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I drive a 06 Subaru Outback (AT, NA). I've had it for about two years. There is a sound which seems like its coming from the bottom of the car. It sounds kind of like the engine revving but when I let off the gas the sounds comes down slowly so I don't think it's the engine but something under the car. A friend told me it might need new differential or transmission fluid. What could it be? Just thought I'd get input here before I take it to the shop later this week. The sound has gotten louder and louder. Thanks.
 

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Exhaust leak? Heat shields?

Need more details about the sound and when it occurs (and doesn't occur), but an exhaust leak or loose heat shield are common sources of sound from underneath the car.
 

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You have two gear boxes that require a heavy gear oil. When the oil gets old it thins down and leaks out. Then you end up with dry gear boxes that get loud just before they fail.

One of those gear boxes is called the Front Diff this is not the same as the Automatic Transmission. This contains gears that drive the front wheels and it needs gear oil servicing every 60K or so. You have a rear diff ie the gear box that drives the rear wheels this also requires gear oil and should be serviced every 60K or so.

If its getting louder and louder and only happens when your moving vs sitting still with the car running - YOU NEED GEAR OIL IN THE DIFFS!

IF you Ruin your front diff you basically need a whole new transmission very costly mistake given a Diff gear servicing shouldn't cost you more than $60 at the local grease pit.

I wouldn't drive the car anymore till you get the diff fluid checked and replaced. Given destroying the diff on an old car makes it potential junk yard material pretty fast considering the repair cost.
 

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06 Outback value is higher than a transmission.

If you know where you are on the scheduled maintenance, start there. The noise could be from low oil, no oil or worn oil in the front differential and what you are hearing is the bearing(s).

It can also be inside the transfer case, which again, could be fluid condition or level related, flow issue, or worn parts.

From your description, it sounds like a bearing issue or gear lash whining. Start with checking the transmission fluid and differential oil levels/condition.

Since you said it gets louder with speed, it is a rotational noise and an indication something is worn or lacking lubrication.
 
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