hi everyone I recently purchased my first Mac from a friend. It had been sitting in storage for a long time. After turning it on for the first time I noticed it had a low thumping sound in both channels with the volume turned down all the way. I've deoxit all pots and push switches. Reseated the driver boards and cleaned the preamp jumpers. It seems like it might have lessened but it's still audible. Pushing in the main speaker switch stops the thumping sound if the speakers are hooked up to main. Same thing occurs with the remote speaker switches when speakers are connected to remote. Everything else works properly. It sounds even and balanced. Just this faint thump sound at all volume levels. Anyone had this problem or have any ideas? Thanks