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Subaru Outback makes loud low humming sound

Hello folks,
This is a question about a Subaru Outback, but probably pertains to many other cars as well. I have a 2006 Subaru Outback with 112K miles. I bought new tires for it in May 2012, and in September 2013 I started to notice a low humming/buzzing sound while driving. I looked online and came up with 2 options - either the wheel bearings or the tires. Because the sound changed pitch depending on the road surface, I thought it must be the tires. I took it in for an oil change and had the mechanics check the bearings. They said the bearings were fine, but the tires were cupping and feathering badly because I hadn’t rotated them, and that was the source of the noise. I had them rotate the tires and do an alignment and went on my way. The sound slowly got worse - it peaks at ~65 MPH and then changes pitch above 70 MPH and is not so bothersome - and I started to lose gas mileage ( 4-5 MPG). On Dec. 30, 2013 I took it in for the next oil change and bought brand new tires - Bridgestone Turanza Serenity designed for quietness - and had them do an alignment, and drove away only to find that the noise is still there and I still have lousy gas mileage. My next thoughts were the transmission or the catalytic converter, but since the noise doesn’t change when I take my foot off the gas or put it in neutral I don’t think it is either one of those. The alignment showed that the Caster on both front tires is off, but since both tires are off by roughly equal amounts I was told that this is not something to worry about. I do occasionally notice some vibration while driving or while idling, but it is intermittent and I don’t think it has anything to do with the noise. My thought is that something is a little off kilter, but have no idea what it could be. Any thoughts? Anyone else have this experience?
Any suggestions would be greatly welcomed.

If the new tires did not eliminate the noise, and did not even change the noise, it is not the tires. I suspect wheel bearings, and the first mechanic missed something, or it could be differential noise or even a carrier bearing in the driveshaft.

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In addition to the good advice from BustedKnuckles, I would advise the OP to have both differentials checked. A differential that is badly worn as a result of inadequate lubrication could produce noise and could also cause enough drag to lower gas mileage.

Although possibly not related to the current problem, I hope that the OP realizes that failure to rotate his/her tires could have led to damage to the center viscous coupler. Subarus, like most AWD vehicles, do not take well to failure to rotate tires, and big repair expenses can be the result.

Subaru specifies tire rotation every 7,500 miles.
How many miles did the OP go w/o rotating his/her tires?

I’ve heard the differential noise described as a whine, and this is definitely a low buzz/hum. I bought the car in 2011 with 86K miles on it and I don’t know if the previous owner rotated the tires regularly. I did not get the tires rotated until I took it in in September 2013 (my bad) with 109K. I had the timing belt changed at the dealer in June 2013 with 106K and they did the full safety inspection and didn’t find anything except that the left front axel boot needed to be replaced and the right had a small tear that could wait.
I plan to take it into the mechanic tomorrow to get it checked out before I drive the 250 miles home next week. My biggest concern right now is whether it is dangerous or not. - and I’m sure it it’s bad wheel bearings I shouldn’t be driving on it…

There is no such thing as a tear in a CV joint boot that can wait. CV Joints can last a very long time as long as they are not driven with a torn boot. Once the boot tears, the life of a CV joint is measured in weeks. However, CV joints don’t usually make a hum. They are known for a loud clicking when making turns, especially low speed turns such as making a turn from an intersection.

Regular differentials make a whining sound when the bearings go bad. Your sound could be from the center coupler and that would be from driving on tires that had more than 2/32" difference in the tread depth front to back, common if you don’t rotate the tires. I hope you replaced all four tires and not just two.

But since your tires cupped badly, there is the possibility of bad struts causing the noise. Its an outside chance but one none the less.

Wheel bearings make a roar and it gets louder when your in a turn. If it is only one wheel bearing, it will only get louder turning in one direction, quieter when turning in the other direction. I would consider a wheel bearing to be unlikely.

My order of likelihood is center differential, struts, CV joints. CV joints first if they are clicking on turns.

My 05 outback had a buzz/hum at about 100K which turned out to be the center bearings of the drive shaft. Which required the replacement of the WHOLE drive shaft.
I also had rear wheel bearing noise which was replaced under Subaru Service Bulletin at about 60K miles.

Well, it turned out the simplest answer was the best (go figure!). It was a rear wheel bearing that the Firestone technicians missed - BustedKnuckles called it on the first shot. Thank you all for your helpful comments. I know a lot more about what can go wrong with a car now than I did 2 days ago and I will definitely take more care in rotating the tires and keeping up with the alignments.

" It was a rear wheel bearing that the Firestone technicians missed "

If you weren’t aware of it previously, this should be your clue that the best mechanics are very rarely found at…Firestone…Midas…Meineke…Monro…Sears…Pep Boys…AAMCO…or other chain operations.

This should be your wake-up call to find a well-reputed independent mechanic in your area, and to drop Firestone and other chain operations like a bad habit. Ask friends, neighbors, relatives & co-workers for recommendations, and also use the Mechanics Files link at the top of this page to find recommended mechanics in or near your zip code.

I will only add a few things. One is that a wheel bearing can be faulty and near undetectable. The bearing may be tight and may feel fine when the wheel is rotated by hand. Once the car is on the ground with weight applied to the bearing the problem may surface yet again.

Two, and I’m by no means defending any shady practices, a lot of those guys who work at franchises work under a heavy boot so to speak and even the franchise owner may get stomped on by corporate. When squeezed, they may squeeze back, or go belly-up.

BustedKnuckles called it on the first shot.
That’s why we keep him around. He makes us look like we know what we are talking about…occasionally

I currently have the same symptoms on my car (Subaru Crosstrek 2013), so I’m thinking of going to my local mechanic to have him look at it, thinking it will probably be a rear bearing replacement job (sound comes from the back of the car at high speed). But considering the vehicle is fairly recent (it has 50k km / 30k miles), I have a few questions :

  1. If it is indeed a faulty bearing, is it normal so soon in this car’s life?
  2. If one rear bearing does show signs of tear / wear and the other one not, can I have only one replaced and not the other?
  3. For an average jo (me) who often does himself simple mechanics on his car (oil, breaks, wheels, filters) would this job be considered complicated or rather simple?
    Many thanks for your comments

Did it wind up being a bad wheel bearing? I have s 2013 Crosstrek with 65K miles and the same symptoms coming from the rear of the car. I had someone look at it. He said it was a bad wheel bearing but, but he couldn’t decipher which wheel it was coming from. I’m curious if that was the source of the sound on your crosstrek. Thanks!

Jan '14
"I bought new tires for it in May 2012, and in September 2013 I started to notice a low humming/buzzing sound while driving."

Oct '16
**"Hi, **
I currently have the same symptoms on my car (Subaru Crosstrek 2013), …"

Feb '17
Did it wind up being a bad wheel bearing?"

Stay tuned. We should have an answer by late 2017 or early 2018.

Please use the “New Topic” button up on the top and start a new discussion. I’d bet you’ll get better results. Give it try. We have technicians and others, including Subaru enthsusiasts, standing by to help. We’d love to have you as a regular here.

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My wife has a Crosstrek and has had wheel bearings up front replaced. I did some research and found that crosstreks and wheel bearing problems go hand and hand. Well, it has been about 6 months when we had the ones up front replaced, now we have to take it in and have the right rear replaced. At first I thought it was the tire as well, but the change in the pitch didn’t match up with a tire problem. So off we go again.