If you cant determine easily where it comes from, just take the whole car apart screw by screw, litterally. It might sound stupid but it's really not. I buy a lot of second hand stuff and a complete rebuild is just mandatory. There is always something wrong , I repeat always. Every time I did it I ended up glad I did, because there was one, two or ten things that needed a rebuild/replacement. Bent axles/ shock rod, gritty bearings, empty/leaking shocks, CVD which had been "lubed" with the MIP threadlock... 98% of the time the diffs need a rebuild.... the list is just endless.
The tc3 is a great car but it needs constant attention, the small AE bearings seem not quite up to the task so they die on a regular basis, esp inside the wheel hubs. The rack needs to be clean at all times. The inner hinge pin will bent just if you look too much at them etc. If maintained properly, this car really is awesome.
As for the possibility mentioned by trixter, it is indeed possible. I had it happen on a 4 cell pan car with a brushed motor on which I had not put capacitors or a shottky diode.