Drivin' me Crazy

I have a buzz coming through my preamp that is intermittent and only through the phono stage. I can tap the preamp and it goes away for a little while but always returns. I have a TAD-150 signature, a tubed pre with two input tubes, which is otherwise excellent sounding. I have changed tubes, inverted my turntable plug and checked and double checked my ground. I also have tightened every screw on the preamp. It seems to get better when I put pressure or weight on the top of the preamp. Any other suggestions?