I have a bunch of API mic pre clones I built. One has become intermittently noisy recently - every so often it lets loose with a rainstorm of hissing and crackling. I tried cleaning the pot but it's sealed, so I don't know if anything got in. I also cleaned the connectors. Could that be the opamp? I have a John Hardy 99C in there now. I realize I could just swap opamps, but I want to understand what's making the noise and why if possible. I will post a link.