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What's my Subie humming for?

My 2006 Subaru Forester has for the past two days, made a higher pitched humming sound when I put the car in reverse and give it gas, before I brake. The sound is getting a little louder. It doesn’t make the sound when I touch the brake, but when I begin to give it gas in reverse again I hear the high pitched tone. It is beginning to do it almost every time I put the 5 spped manual into reverse. I drive many miles each day, the car has 85,000 miles on it. Any idea if it’s safe to drive? What’s wrong with it? I have to go to work in the morning and am concerned.

Is this noise coming from up front; say around the center of the windshield, etc.?

If so, the first thing you should do is check the gear oil level just to make absolutely sure this is not a ring/pinion gear whine, etc.

Had the engine oil changed very recently?

The hum is definately coming from the front, maybe even a tire. Thanks for the tip, we’ll check the ring/pinion gear.

The oil is due for an oil change now. I’ll be taking it into the shop soon, just wanted to have some idea before I went. Thank you for your help. I appreciate the information!

I just mention the gear oil thing because a ring/pinion failure is a major and expensive one. The reason why I mentioned the engine oil change is that it’s not unheard of for someone to remove the wrong drain plug when performing an engine oil change and empty the transaxle by mistake

This kind of noise will usually be present in the front center of the car. If this noise is coming from one side or the other then it could be a tire tread or wheel bearing problem.