Converter broken???

Hi, I have a problem:
I have a TH700 trans behind an almost stock 350 Chevy SB in a G20 van. When in Overdrive there is a very noticeable reverberating sound and feeling. It sort of goes VRRRMMM...VRRMMMM...VRRRRMMMM with one VRRRMMMM about every second or so, depending on the vehicle speed. If I take my foot of the gas the sound stops, so I guess it's not an unbalanced wheel. Engine and trans and driveshaft are new, just the converter is still the old stock one. It had that sound before the restoration, so I guess it can only be the TC. Is it possible to fry a TC? If yes, then how can I keep it from being fried again? My predecessor (only bought the vehicle a short while ago) fried the trans completey by not-using a TV-cable :-( Can you burn out the OD-lockup-clutch in the TC? Never heard of that happening...
Please help me out it you know something about TCs.
Thanks a lot,
Mad

There's a bearing in the converter that's shot. A rebuilt one is in your future. Its not uncommon for them to start vibrating like that.