Haha - most amusing! ๐
It's in a chain of pedals which come out of the Fx loop of an EBS Microbass 3.
My test will be to put something else in its place and see if that has a similar (ahem) effect.
Yes, I'm thinking along those lines.
I'm also going to try putting an MXR Noise Clamp around the Fx loop (though it too is buffered), to see if that cancels it out. It does use up a pedal space, though.