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I have a 05 WRX which has been producing grinding noise from somewhere between the center to the back of the car. The noise starts to happen once the car is warmed up and under power. It can also keep going when lifting the foot and coasting as well. The transmission is a 5MT and still shifts smoothly. The noise usually happens when driving over 50 kph.

I changed the fluid from the rear diff. The magnet on the plug didn't have any metal attached to it. The fluid was dark but no sign of metal shards. Putting new fluid in the rear differential didn't help solve the problem.

Now last weekend I started to notice rear wheel hop when performing tight low speed turns when parking the car.

Can this be the center diff getting worse? What is the best way to get to the root cause of this issue?

Thank you

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I'm not sure on the Subaru, but it sounds like your transfer case is having issues. When a transfer case is locked up, it can cause the wheel hop you're talking about. This is very common with vehicles which have a regular switchable transfer case (like my Chevy Silverado). On my truck, when in regular 4wd (not auto-4wd), if I go to turn the wheel while going around a corner it will ... er ... lope. I asked the Chevy dealer about this and he stated it's because the transfer case is loading up, which is not good for it. In your case, since yours is full time AWD, it sounds like the problem is in your transfer case and it is behaving like mind does in 4wd.

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  • Thank you Paul for your answer. the transfer case is analogous to the central viscous coupling diff on Subarus. I had a long chat with the service dept from my local subaru dealer. They are proposing to troubleshoot it on a hourly basis until they find the problem which might be very costly. The car is obviously out of warranty. I was told I would still be able to drive the car for a while and monitor symptoms. I will post the steps they took to troubleshoot the problem when done.
    – Jean-Christophe Rodrigue
    Jul 18, 2019 at 22:17
  • So the transmission/transfer case gear oil was changed. The used one poured out like water... New one went in and it seems it has reduced this issue greatly.
    – Jean-Christophe Rodrigue
    Oct 9, 2019 at 22:11
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So the weird grinding noise was caused by the exhaust expanding due to heat. The expansion made an exhaust hanger touch the hitch that is installed on my car. Once these touched the engine vibration transmitted from the engine through the exhaust resonated with the hitch.

The noise problem had nothing to do with drive train components.

The wheel hop is unrelated to the noise but has been confirmed to be the center differential. A new center differential has fixed the problem.

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