rear suspension chatter?

I seriously doubt it, but it could be the rear end backlash. When test driving my GT, and my father was driving ( it had been 20+ years since he drove stick), the rear end clunked when shifting. When I drove it, the clunking wasn't there.... you just have to know how to drive a manual tranny.

Like I said, I seriously doubt that's your problem. But if you happen to be hearing more of a clunk from the rear diff that a "really loud knock", then it may be the rear gears. All depends on how well you shift.
 
this may involve some tools

make sure that your shocks are tightly bolted down at the top of the mounting point... you need to pull back the trunk linning and i think its like a 13mm and also make sure the bushings are not deteriorated... my passanger rear shock was loose and i got a slight clunk at times... let us know what you find please