... I'm the chap that BassAdder said could keep the unit due to the above noise issue (thank you again, very kind of you).
I did read a thread a while back on here that indicated that there has been a noise fault on some units (confirmed by Keeley themselves I think). On my various setups, most apparent on this example is when using a passive bass (Zoot Funkmeister 5), the noise being really very obvious with the settings as per normal use (gain and threshold not at extreme settings at all, I do use IEMs though) on both short staccato and long sustained notes. However the same Zoot bass with another compressor (I have a few, think I used an Empress Bass Compressor for comparison) and no noise issues at all.
I have worked out via trial and error, that if I use a buffered pedal before the compressor for comparisons sake, the noise issue is reduced, but still audible to me. Potentially a faulty batch of components used in manufacture, or maybe this particular unit wasn't calibrated as well as others from the factory, I've really no idea? I've chatted to a friend (LukeFRC above) about this before and he came up with this link:
https://robertkeeley.com/blog-post-archive/threshold-indicator-schematic-board/
I've had the back off and couldn't see anything as described in the blog post as it'd have to come apart to see the other side of the PCB. I've no idea if it's something a non-specialist could adjust.
Either way, thank you again for the kind gesture BassAdder.
Also sorry Rollin Thunder if this reply has derailed your original post, glad you're digging it 👍