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Bearings or Rotors?

Hello, everyone!
My car expertise up to now has been oil changes and brake changes.

For about the last two months there has been a dull drone coming from the rear of my 2015 Subaru Forester that just hit 100,000 miles. The drone does not get significantly louder with an increase in speed and does not get better or worse if the car is turning left or right.

The tires have been changed on the vehicle and the drone has remained unchanged on both sets of tires.

Today I had jacked up both rear wheels and found that when I attempted to spin them, they BOTH made a grinding noise as they spun. There was zero play in either wheel and both rotors have visible rusting on their outer edges.

I am far from an expert in fixing cars and am looking for your opinions.

Is it possible that both bearings are bad despite there being no play in either wheel?

Could my rotors be causing a noise that mimicks that of a bad bearing?

Are there any other potential causes for this that I am not considering?

Thank you very much for your time and assistance!

Mike

Where was the grinding noise coming from?

It was a sort of sandy grinding coming from both rear wheels, independently, when I hand-turned them while the car was jacked up.

I bet it’s rear wheel bearings, a common problem at these miles on a Forester. They can check the brakes as they’re taking things apart.

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So, as a follow-up question and keeping in mind my lack of experience here: how could I know definitively if it were one or the other?

All of my internet research suggests that bearings are not replaced in pairs which leads me to believe that it is uncommon for two of them to simultaneously fail.

What would I be looking for if I were to remove the wheel / what tests could I perform to rule out one or the other?

Thanks for the responses! Very much appreciated.

By using these

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Or these

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Are you planning to do the repair? I’d find a good subaru shop and have them do the diagnosis and repair.

I’m comfortable with brakes and rotors, but, have never attempted a bearing replacement.

My thought right now is that if the diagnosis isn’t clear cut to just to replace the rear rotors and brakes to see if that fixes the issue.

If it’s still creating that noise, I’ll have narrowed down my search, be out a minimal amount of cash and have some new equipment on the car.

I doubt it’s brakes and rotors…If you’re comfortable with fixing brakes a quick inspection should give you the answer you need.

Jack up one rear wheel, remove wheel ,caliper and rotor. Spin hub. If noise is still there it is not the brakes.

Since the noise seems equal on both sides, could the source of the noise be upstream from the wheel bearings - transfer case or drive shaft or ? - and heard at the wheels?

Why keep guessing? @db4690 showed the right tool for definitively diagnosing these type of noises. Probably cost less than the first guess…

Subarus don’t have a transfer case.

Noted. I live and learn. Drive shaft? viscous coupler? or ? Whatever they call the components between the transmission and the rear hub bearings, could the common source of the noise be there somewhere?

If we ever get a break from the constant rain here, I plan on checking this out for myself. My 2014 Legacy is doing the exact same thing, It also started at around 100k miles. BTW, the volume of the noise does not change with speed or with turning, but the frequency (pitch) does change with speed.

I both front wheel bearings on my 2002 Saturn go out at exactly the same time at 274k miles so it can happen. One rear wheel bearing was bad but I didn’t notice it until around 84k when I changed to new tires that were significantly quieter, but the other rear never went bad.

BTW, they don’t look hard to replace. You have to replace the whole hub assembly. The hub assembly looks just like the front hub assembly on the front of my 07 Silverado 4x4. It was easy to replace.