You step on the accelerator pedal, expecting the car to purr like a kitten as it whirrs up to speed, only to find a strange humming noise when accelerating. The hum might be soft and simply sounds a little out of place, or it might be so loud that you can barely hear yourself think.
Some of the top reasons why your car makes a humming noise when accelerating could be related to one or two bad wheel bearings, uneven tread on your tires, a transmission issue, or a problem with the serpentine belt.
To help restore your peace of mind and the quiet running of your car, we’ll need to delve into the top reasons why a car makes a humming noise when accelerating, how to confirm it, and how to fix it.
Why a Car Make a Humming Noise When Accelerating?
Sometimes, the humming noise can come on so subtly that you don’t really notice it until it becomes a problem. Some reasons why a car hums when accelerating can come on fast and might be so serious that you need to pull over as soon as possible.
Top 5 Reasons For Car Makes Humming Noise When Accelerating
1. First Look for Bad Wheel Bearings
One or more bad wheel bearings are one of the most common reasons why a car makes a humming noise when accelerating. The wheel bearings connect the wheel to the axle, allowing it to turn smoothly. They start going out slowly, making a soft humming noise that you might not immediately notice. As the bushing housing starts to break down, the noise can become suddenly loud.
To the point where it can be hard to listen to the music or hear yourself think with the hum of bad wheel bearings in the cab of the car. It can also cause severe and dangerous wheel damage.
Signs of Bad Wheel Bearings
How to Fix Bad Wheel Bearings
If you don’t have a lot of experience with wheel bearings, then it’s probably best to have a mechanic replace them. Especially if you don’t have the press necessary to pack replacement bearings in the bushing. Usually, when one bearing is worn out, the other one on the same axle is about to go as well. So, it makes sense to have both done at the same time.
The cost to have a mechanic replace one wheel bearing ranges from $225 to $350 per wheel.
2. Uneven Tire Tread can also be the reason
Uneven tire tread from an unaddressed alignment problem, overinflation, underinflation, or worn-out suspension components can cause the car to make a humming noise when accelerating. If the tread is badly worn, the sound can be mistaken for bad wheel bearings. Though usually, you’ll notice dips, scallops, or other deformations in the tire.
Signs of Uneven Tire Tread
How to Fix Uneven Tire Tread
When uneven tire tread gets to the point where the car makes a humming noise when accelerating, the tires are too far gone to save. Usually, you have to replace both tires on that axle.
If an alignment issue caused the uneven tire tread, you will need to have it corrected as well, or the new tires will simply start to wear down the same way again. If the uneven tire tread causing a humming noise when accelerating is due to a suspension system failure, the cost might be significantly higher.
The average cost to replace tires ranges from $99 to $175 per tire, depending on the size and quality of the tire.
The cost to correct the front-end alignment of a car ranges from $100 to $250.
The cost to replace the front-end suspension on a car can range from $575 to as much as $2,500, depending on the components that need to be replaced.
3. A Problem in the Transmission
Several different transmission problems can cause a car to make a humming noise when accelerating. Though most are related to excess wear on the gears, low transmission fluid, oxidized transmission fluid, or a mechanical fault deep inside the transfer case.
One of the things that distinguish the sound is that the humming noise from a transmission issue tends to sound like it’s coming from the floor of the car. In a scenario like this, you should start by checking the transmission fluid. If you’ve recently overheated, or you had a long drive towing, or the car has been running hot, the transmission fluid might have broken down, turning dark.
Signs of a Transmission Problem Causing Humming Noise
How to Fix Transmission Humming Noise
If you’re lucky, the problem is only limited to the transmission fluid, and you caught the problem early, then you might be able to stop the humming by topping up or replacing the transmission fluid. This is only a reliable fix if the transmission fluid is low or dark and without metal shavings.
The average home mechanic can usually replace their own transmission fluid. Though it’s also something that you can usually have done at an oil change service center for around $75 to &125.
If you have metal shavings in your transmission fluid or the car makes a loud humming noise when accelerating, then you may have a serious problem with the transmission itself.
These repairs are always expensive, and the floor cost to have a professional mechanic diagnose and fix a bad transmission starts at $1,500.
4. A Faulty Serpentine Belt might be the source
Improper tension in the belt drive system that holds tension on the serpentine belt can also cause a car to make a humming noise when accelerating. This is even more likely to be the cause if you’ve recently replaced the serpentine belt.
It could also happen if you recently had to work on something under the hood that required you to take the serpentine belt off and then retain it. However, some cars will get a humming noise and whine from a worn-out serpentine belt.
If any of these things are possible, you should check the serpentine belt for tension issues and signs of excess wear.
Signs of a Serpentine Belt Problem
How to Fix a Serpentine Belt Problem
If the belt is a new replacement, or you’ve recently had to take the belt off and put it back on as part of another repair, you might be able to adjust the tensioner arm simply.
If the serpentine belt is frayed or showing signs of wear and tear, then you likely need to replace the belt to stop the car from humming when accelerating. Both of these are things that the average DIY mechanic can do at home. Though there’s no shame in taking it to a mechanic if you’re short on time or feel you don’t have the skills.
The cost of a replacement serpentine belt is usually around $35 to $75. You should be able to replace it in under an hour.
The cost to have a mechanic replace your serpentine belt ranges from $75 to $175.
5. The alternator can be the potential cause
If your car’s alternator has a failing rotor, it might also make a humming noise when accelerating. This is usually due to the bearings wearing out and affecting the movement of the rotor. This can also affect the alternator’s ability to charge the battery.
Signs of a Bad Alternator
Most auto parts stores will test an alternator for you for free. Diagnosing bad alternator bearings usually requires a bench test, where you have to take the alternator out of the engine bay.
How to Fix Bad Alternator Bearings
Usually, when the alternator bearings start going out, causing a car to hum when accelerating, other faults within the alternator need to be addressed. You might be able to have a mechanic rebuild the alternator, but it’s usually better to completely replace it.
The cost for a replacement alternator ranges from $120 to $300, depending on the make and model of the car.
The cost to have a mechanic replace your bad alternator averages around $325 to $425.
Frequently Asked Questions
Is It Safe to Drive a Car That Makes a Humming Noise When Accelerating?
A lot of the problems that cause a car to hum when accelerating are related to serious mechanical or functional faults. You might be able to limp home or to the mechanic, but no further.
Bad bearings can cause a wheel to tear free from the car, tires with abnormal tread wear are prone to catastrophic blowouts at speed, a bad alternator can leave you stranded, and transmission problems get incrementally more expensive with time.
Is It Normal for a Transmission to Hum When Starting Out?
When you’re first starting out from a stop into first gear, a little hum from the transmission is normal. This is usually just the valve body sending pressurized automatic transmission fluid to internal bands and clutch packs. It should stop within a few seconds after taking off and might last longer if you are at a stop and starting out at a steep incline.
How Can I Prevent Treadwear on Tires
Preventing abnormal treadwear and scalloping on tires starts with making sure the car’s front-end alignment is good and the tires are properly inflated. Making sure to rotate the tires per the manufacturer’s instructions will also help maintain even tread integrity on your car tires.
Anytime a car makes a humming noise when accelerating, it’s trying to tell you that there’s something seriously wrong under the hood or with the wheels. The most common reason is often bad wheel bearings, though abnormal tread wear on the tires, serpentine belt problems, and even transmission issues can also be afoot.
These are all issues that can blow up in your face if you keep driving a car that makes a humming noise when accelerating. If you’re lucky, you might only need to replace oxidized transmission fluid, adjust the belt tightness, or balance the tires for around $100.
If it’s a more serious issue, such as multiple bad wheel bearings or metal shavings in the transmission fluid, the cost to fix the humming can go up quickly. It’s usually smart to replace both bearings on an axle at the same time, which can run you upwards of $500 or more.
If the reason the car makes a humming noise when accelerating is related to a serious transmission issue, the cost starts at $1,500 and can go up sharply from there.
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.