Boss CE-1 Low Frequency thud/buildup problem

[richppp]

Hi, new here, i've had a look around and couldn't find any similar problems - i have a working Boss CE-1 that has a very low frequency "thud" on the output especially of the chorus section, but also on the vibrato at certain intensities and rates (not fastest but medium to low). It is 0-50hz on an analyser, obviously shouldn't be there as is clearly audible, and in time with the rate/LED. I've moved a few of the cables to the leds to see if its interference but no change. The transformer (uk voltage) was humming but by temporarily moving it from the case it looks as if it's a physical vibration rather than a failure.

I'd like to learn also how to think through these problems and hopefully tackle them myself, I have safe soldering skills and can mostly read a circuit diagram but dont really have the electrical knowledge how there's potential for interaction between components that might be unexpected rather than following paths etc. So any suggestions welcome.

My initial thoughts were this was either a power supply issue and the intensity is amplifying some mains interference or the opamp for the chorus circuit is faulty - it is not socketed so before i start randomly changing caps or chips thought i'd ask . it has obviously been opened before, there are some dodgy transistor replacements just soldered onto cut legs, and a joined capacitor in place of a resistor near IC4 ! Again before returning these , as the unit works, i'd like to tackle the thud first

thanks if anyone has experience with these.