Oddball454

I am having some trouble with the factory stereo in my 95 T/A. It originally had a tape deck style head unit and it started to make a buzzing sound in the speakers when I would go over bumps. It started with the passenger side rear speaker, so I replaced the rear speakers with non oem speakers. The problem went away for a little bit and then came back, so I unplugged the speaker and it seemed like the problem went away. (When driving with the top down and the radio up louder the buzzing isn't easily heard.) It then came back to the drivers side door speaker. I didnt think this was a coincidence so I replaced the head unit with a factory CD player style. The new head unit did not fix the problem. The car had an amp in the trunk when I bought it, which I fed a ground to directly from the battery to the amp to see if that was the problem, and it wasnt. I also fed a direct ground to the head unit, and still no good. The buzzing doesn't happen all the time, but seems to only happen when I go over bumps. To get rid of the buzzing I either need to go over more bumps (sometimes it goes away, but sometimes it comes right back), turn the volume up really loud and then back down, actuate the power locks or windows, or turn the radio off and then back on. Has anyone else had a similar problem, or have anything else to try??? I'm finally stumped with this one.