Grounding A Preamp

I have a NAD 1155 preamp with a nasty buzz in the phono stage. It's not a phono grounding issue, because I checked that. After the preamp is on ~40 minutes, the buzz goes away after I touch the faceplate. So I am acting as a ground. Should I just run a wire from the preamp to a electrical outlet screw, or what? I am at a loss.

TIA,

Lee