I have done so many different things that my brain flushes the small details out to make room for what I am currently playing with - I hesitate to pontificate with half remembered facts.

Hums and buzzes in electronic circuitry are often caused by "Earth Loops". This is a tricky subject in itself, but there is lots of info out there explaining it and suggesting fixes that the budding sound engineer can search for on the net.

Basically, it is caused by an AC voltage difference on the earthing points between connected sound modules, including long runs of cable. Simply put, you avoid it by making sure that you control your earthing points - and I am talking 'signal earths' here - DO NOT TAMPER WITH MAINS EARTHING UNLESS YOU REALLY KNOW WHAT YOU ARE DOING - you could kill yourself. You can get interaction between 'signal earths' and 'power earths' too - commercial items SHOULD have set up the power earths correctly, and also the 'signal earths' WITHIN each item.