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Discussion Starter #1 (Edited)
Last week I noticed a buzzing sound coming from the rear of the car. I am trying to figure out what it might be and how to tell for sure before trying to fix it. Searching the web I get that it's either a wheel bearing or the rear differential/central driveshaft.

Here is what I know. Back in October (about 4-5k miles ago) I hit a curb and damaged the front right tire. When I took it in to the tire place they said the tread depth was right at the 2/32" difference restriction. I think it was just a hair more. The new tire was 11/32" and the other tires were at about 8.5/32". The tire dealer said that it should be fine. They were aware of the Subaru recommendations on replacing tires and since they had incentive to try to sell me four tires instead of just replacing the one I figured they were being honest. So this does raise the possibility of the rear differential wearing out prematurely.

Right after the tire being replaced I noticed a pulsing humming sound coming from the front right wheel above 40mph that goes away when I turn right and gets worse when turning left. I suspect that is a wheel bearing.

Now I am hearing a new sound that sounds like a buzz at about 25mph and can't hear it as well at interstate speeds. I have not noticed a change when turning. It does sound as though it may be coming from the left side. My finances son sat in the middle seat in the back and said he also thought it was coming from the left side behind him. The sound only changes with speed and not acceleration or rpms. Putting it in neutral also makes no change in the sound.

Is there some other ways to determine for certain what is the cause? My dad has a lift I could use to lift the car and put it in gear if that might help diagnose it.

If it is the rear differential, what would likely need to be done to fix it?
 

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Just a question. Do you still hear this buzzing from the rear left with the engine off ? If yes it could be the evap system. Grasping at straws here.
 

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Discussion Starter #3
I only hear it when in motion. I wonder if I can put it in neutral and turn off the car while in motion. Obviously someplace in a safe location to do so.

However, I did find in the user manual that the cars AWD light will begin flashing if it detects tires of differing sizes, so whatever is going on may not be because of the newer tire.
 

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Mine was a wheel bearing (115k miles on my soob). My test was jacking the car up and rotating the tire back and forth. It should be virtually noiseless.


Sent from my iPhone using Tapatalk
 

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Discussion Starter #5
Mine was a wheel bearing (115k miles on my soob). My test was jacking the car up and rotating the tire back and forth. It should be virtually noiseless.


Sent from my iPhone using Tapatalk
I'm hoping it's just the wheel bearing. I need to lift the car and try turning the wheel soon, see if it makes noise. I think the right front one is bad due to what I mentioned in the OP, but so far I haven't heard noise when I rotate manually.
 

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Discussion Starter #6
Ok. Just got the car up on a lift and tried to listen while someone pushed the gas. First, is there a good way to do that without the traction control constantly hitting the brakes. It was a little unnerving for the car to keep shuddering when it would try to brake. Second, here is what I noticed. Before I turned the car on I noticed both back wheels were a little harder to turn than the front, and made more noise too. Not quite a grinding noise but like the brakes were applied a little. Removing the brake pads made no difference.
When hiring the gas I think I heard the buzzing. Using a stethoscope I couldn't hear anything from the rear differential. I also noticed that when in gear all four tiers would spin but the left rear tire would spin much more slowly.
Any ideas here? Right now I'm thinking either both wheel bearings are going bad or there is resistance in the differential, but I couldn't hear anything from the differential using the stethoscope.
 

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Discussion Starter #7
Put the car up on a lift again and started checking things. I realized that cars with rear disc brakes also have a built in drum brake for the emergency/parking brake. I adjusted those to loosen up and It fixed the issue of resistance and grinding noise when turning the wheels. However it did not fix the buzzing sound. It is still there. Also I am not sure if this is normal but I discovered that when in park, if I rotate a rear wheel the other wheel does not turn unless I lock the drive shaft going into the differential. Then the other wheel will turn the opposite way. Is this normal for the rear wheels, the front wheels will turn opposite without having to lock anything (unless that's what putting it into park does is lock something for the front).

So now it seems the wheel resistance and noise may not be related. Also means the wheel bearings are likely ruled out. Any ideas? My brother thinks it's still the emergency brakes.
 

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My 2014 also sounds like the diff is humming. Anything over 15- 20 mph you can hear it start up. My old 2001 impreza had the same kind of noise just a bit louder. I'm thinking maybe this is just normal, although, it shouldn't be.
 

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I took mine in to get it checked out. The reason it sound like it was centered is because both rear wheel bearings were bad. That's 3 of 4 so far in 90,000 miles.
 

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I have a 2015 CrossTrek with manual transmission. The humming started about 2 weeks ago. I only have 48k miles on it. Seems very early for a bearing failure. Yes the hum varies with car speed not rpm, and it starts around 20 mph. Sounds like the hum of a bad differential but I see no leaks causing possible fluid loss. Has anyone had Subaru do a diagnosis?
 
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