I have a 70 chevelle that has a loud vibration around 4500 rpm. It sounds like its coming from under the seat ,"possibly the driveshaft" I was wondering if anyone else experienced this problem or has any other ideas. The sound comes and goes lasts a second goes away a second and keeps repeating until the rpms go down.

When was the last time you checked your transmission mount? A $10 item if necessary, depending on material (poly/ureth/rubber).

check the u-joints

And, before you do anything, get it up up on jackstands and check the security of the U-bolts that hold the driveline in place, check the crossmenber bolts and rear transmission mount bolts, and if everything is tight, then move on and inspect the u-joints, rear trans mount, and motor mounts.