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?