VPI PLC ground/hum problem--HELP

Just purchased a used PLC to use with my HW-MKIV/SME309/Sumiko Celebration and have encountered a very noisy and persistent ground/hum problem that renders the PLC unlistenable!! The hum varies in pitch with the speed controls and thus far I have attempted a variety of component placements, lifting the ground on PLC, changes of outlets, etc. Does anyone have some experience with this problem and a possible solution? I did not receive a manual with the PLC and also wondered what the trim pot on the back was used for?
You may not be experiencing ground hum. The PLC II is noisy. It vibrates right along with its transformer, audible above much of the sound. Put large sorbo feet under the unit and a VPI brick on top. That knocks some of the noise out, but leaving it on all the time helps even more. Just removed the PLC II from the system and replaced it with a Walker Precision Motor Drive. Guess what, I can hear it running, too. Had to put Walker discs on top and points underneath to quiet it down. The transformers and/or motors make these things noisy. Hope someone out here on Audiogon has a better way to calm these things down.
These PLCs weren't the most reliable units in the world. VPI couldn't fix mine (bought used) and sent a brand new one - great customer service. I'd give Mike at VPI a call (Tues. thru Fri.). Better yet, I'd pick up an SDS. It's better in every way. Good luck.