r1pilot

I have a 93 SC400, I need a little help trying to find the cause of a vibration/rumble sound that is both heard and felt but only at about 2500rpm when sitting and in park or at the same rpm while driving. the car accelerates fine but won't go into top gear until around 80mph if you push it. I want to think that this is as simple as a motor mount but I'm already making a contingiency to purchase a new tranny. The idle rumble sort of drones if you hold the rpm and when cruising on the expressway sometimes it does it sometimes it does'nt. I know somethings not right and I also believe you all can give me some ideas. So thanks in advance. If I don't respond right away it's because I leave sunday for Cancun and won't be back for a week so please leave your responses and I'll have all your suggestions in the "LAB" as soon as I get back.