I'm experiencing a puzzling issue with this 688 midistudio. Everything seems to work fine, except an intermittent popping through channels one and two, when no inputs are used and no tape is in the deck. The sound can be heard through the monitor and the meters bounce accordingly. I've got electronic repair experience, I'm just looking for tips to speed up the troubleshooting. So far I've cleaned up all the pots, faders and connections. I've also cleaned the heads of the tape deck. I'm thinking this it's an output issue, because signals are good on their way into these channels. Any advice would be welcome. I'm new to tape machines.