Mcintosh mc2505 has a hum on the transformer......

99% it is normal. the screws or the nuts on the transformer mounting tends to be loosen-up within the certain time. you should tie them up as tight as possible from time to time and you will decrease the hum significantly if that's the case. you could also use some heavy load on the top of the power transformer. i use the following: felt self-adhesive stickers placed on the surfaces of both solid puck-shape piece of steel(arround 800grams) and the top of the transformer and than stick them together.
the hockey pucks underneath the amp's feet can also help to isolate from the floor and absorb the transformer vibrations.

I wouldn't worry too much on that issue than if it's not audiable with music.