i would try the amp on another outlet or a cheater plug,i owned a mc7104 & it was dead quiet,if its not some problem with your power source then i'd say it needs service,there should be no noise at all from a 7104.
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I have never heard a McIntosh make any sound. There could be two simple problems - the transformer got loose in shipping or the grounding is bad. The latter could be a loose chassis ground or your outlet/circuit/house has a bad ground.
You could take the cover off and check these things very easily. It should be rather obvious. Also, Joe is right about trying out a cheater plug.
As stated above, you should contact McIntosh Labs with your question on the physical or mechanical hum coming from the transformer on your MC7104. If your unit does not turn on or if your situation is an emergency, do not e-mail or fax McIntosh. Phone the factory, ask for Tech Support and leave a voice message for Chuck. He will return your call promptly.
Chuck Hinton: firstname.lastname@example.org
Hours: 9:00 am - 5:00 pm EST, M - F
Toll Free: (800) 538-6576
Phone: (607) 723-3512
Fax: (607) 724-0549
If you have a DAC or other digital equipment they could be causing this problem. Some brands will not work well with others and there is nothing you can do, because it is air borne. It could also be getting some DC voltage off the main power line from something in your house and again a good power conditioner(not all are good at removing DC or a ground loop no matter what they cost), but I have found BPT and PS Audio to be the best for these problems, but I am sure there are others too. Hopefully it is just loose.
Jesus Is The Way,