You might be hearing the motor noise/vibration being picked up by the needle. I'm not familiar with your Garrard, but judging by its age, it's probably not designed the same way that modern higher-performance turntables are. Most are designed so that the motor is either isolated from the platter and drive system or at least decoupled or suspended so that there's little or no vibration.
It might be some sort of ground loop issue, though it sounds like you've explored that possibility. It might also be your receiver or AVR. Some of the less expensive gear doesn't fare well with turntables...
If you're really enjoying the vinyl, it might make sense to shell out a couple hundred bucks on a decent entry-level turntable. I'm still up in the air as to which one I'll purchase, but I'll certainly post once I make up my mind.