Noisy in gear

Just got a 1990 88 and it runs great and sounds great when not in gear. When I shift the car into gear it makes a sound like a rock in a can. This only happens when you give it some gas either in drive or reverse. The car shifts fine and has plenty of power it just makes that weird noise and I cannot figure out what it is. It only happens up to about 35 mph. Any ideas?

Welcome to AF, many things can cause this, someone knowledgeable should drive the car for an accurate diagnosis, not possible from here of course. If it does it just power braking, it could be a flywheel or convertor problem, only guess work here.