Motorboating Java Boost question

[rockhorst]

I've got a Keeley Java Boost (the blue one with cup o coffee on the front, not DIY) here that motorboats when Idle. It didn't do so before, but after years of service it suddenly started doing this. You can't really hear it while playing but as the note dies out, in comes the steady brm brm brm brm motorboating sound. None of the following made a difference: disconnecting the LED, cap over power supply, sanding paint from jack holes for better ground or a different transistor. The strange thing to me is that is motorboats EVEN when powered with a 9V battery.

One thing that makes it almost dead silent is putting a buffered pedal like a Boss in front of it. Motorboating gone. The trade off is that the treble booster now is instant ice pick instead of that sweet sound with no buffer.

I didn't have this with my own RM type clones, but I'd like to understand what's going on here. Maybe aging electrolytics? Any other suggestions?