For the last few months my 2002 WRX has been making a knocking sound every once and a while when driving slow such as in a parking lot. The sound occurs every few seconds and seems to be on the drivers side of the car. Additionally, when I reverse up hill I hear a rapid knocking sound from the center of the vehicle.

I originally thought this could be the strut tops, or a wheel bearing, or a ball joint but they all seem to be fine.

Yesterday I gave up on self-diagnosis and took the car into a mechanic. They put some wireless microphones under the car and tracked the sound to the transmission and belive that the front differential is making the noise.

Before I had the transmission torn down I want to cover all the bases. Can anybody suggest any other possible sources for this noise? It happens whether or not the clutch is in and seems to be a little worse when the car warms up. Could bad cv joints cause a knocking only in reverse? I don't see any wear points so I don't think the exhaust is hitting.

Some stats:
- 2002 WRX w/ manual transmission
- 71k miles
- Kartboy short shifter and bushings
- v7 JDM takeoff struts w/ Prodrive springs
- Rebuilt front calipers about 5-10k ago trying to fix noise, but it didn't help.

Any help is much appreciated.

- Matt