Hum and noises in McCormack amp

I just picked up a used McCormack micro power drive amp. The amp sounds amazing. However the amp puts out some hum when not attached to anything except the speakers. And a little bit of scratchyness in the tweeter.

But then when I attach the front end it hums even more and also makes a noise that sounds like dut..dut dut ....dut dut dut dutdutdut.... dut dut. etc. And a crackly noise a little more in the tweeter.

Its noticible enough to be annoying during quiet passages or when the music is stopped.

When I borrowed an adcom amp it was dead silent. No problems at all. Dead quiet and smooth. Headphones are thes ame. But I just can't seem to get this McCormack to quiet down no matter what I do. Is it possible that it's just old and circuits have gone bad (or whatever else is in there)? Or does it just not like my home? Either way I can't live with it like this. I've had it a week so I'm sure I can still take it back to the store. Although I did tell them the amp sounds amazing and I'd hate to walk in and say "sorry I can't live with it now". Its done this since day one but I thought I could tweak some things to improve it and it's gotten more noticable since I've improved my room and made the room quieter. (and gotten to do more extensive listening).

So do you think that something is just bad or aged and can it be fixed by the store? Or should I just return it and get something else?