Could be a lot of things, but probably a microphonic or leaky cap or a bad foot switch.
You can test for microphonic parts by tapping around inside with a chopstick (if it’s a deluxe model with a transformer then be careful of the mains connections) and you can try cycling the foot switch on and off which might point to it being the problem but otherwise it might be best to send it to a tech.
I had the same problem happen with a vintage Frequency Analyzer pedal. It sounded good aside from the gunshot sound whenever I’d turn it on. No idea what was causing the problem though, as I ended up just returning it instead of having it repaired.
