Slight hum in my amp. Is this normal?

I have an Anthem A5 and when I turn it on there is a slight "hum" if you put your ear close to it. From 3 feet away it’s unnoticeable. It's in the amp, not in the speakers.
Is this normal?
I don’t recall ever hearing it before. When I moved into a new home, the right surround was dead. I took it to an authorized Anthem service center, the only one in the USA, they couldn’t find any problem so as a preventative measure, they disassembled it and reseated everything. When I powered it up is when I noticed. Note this home was built in 1941 and still has "knob and wire" electrical. It looks like everything is grounded but electrical is not my specialty. I’m purchasing a Torus TOT MAX tomorrow so I wonder if that will help. I'm not sure if everything is sharing the same ground. If it were a grounding issue, wouldn't I hear it in the speakers? This is not coming from the speakers, only the amp.
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I've always experienced power transformer hum and HF hiss until my current ARC and Hegel H30 amps.  Both dead quiet so I guess it's design dependent.
Question for oldschool...if you've got 'knob & tube', I can only assume you don't have the 3rd prong 'direct ground' wiring.  That could be an issue, even if it's all wired correctly.  In order to use those wall plugs which Should be 2 prong only, you'd be forced to either use a 'dead man' 3>2 plug adapter at each connection for such, or been doing some 'plug surgery' and snipping off the 3rd prong.

In the first case, the 3rd is just hooked up to the normal ground.  In the second, you just 'sidestep' it, which does eliminate any common grounding between any and all chassis ground connections.

The 3rd dedicated ground exists solely for short-circuit safety issues, and it's what a GFCI depends upon in the modern kitchen and protect you from grounding yourself out if you come in contact with water if using a faulted device.  By extension it works for the balance of the home if, by chance, you're operating a faulty appliance or spill your drink on some device.

Now....given your situation, if there's no hum exhibited from any other device, within your system or without (a fluorescent lamp or some other device being run concurrently when you've powered up), and it ONLY exists from the one device that you've had refurb'ed, I'd look at a interconnect cable going into AND out of the amp.

I chased a 'hum' gremlin in my system not long ago, swapping power cords, power sources, interconnects on All components, hunting down a somewhat loud hum.  It finally came down to One faulty cable of a pair, which had either a bad RCA plug or a fault in the wire itself.

Viola', it worked.  To be honest, there is a slight residual hum from the amp if I crank it up with no program.  And a power conditioner may or may not minimize that, depending on (as noted) the design and parts used in your amp and other segments in the signal chain.

While I'm being long-winded and pontificating *G*, you might try hooking up the amp into a 'bare bones' setup....different speakers, one source, something really minimalist.  See if it still hums at you.  Dig out the 'orphans' in the attic, closet, or garage...hit up a buddy for an afternoon 'test', you spring for the lunch and libations...

Not that I'm not telling you to buy a power conditioner, but it'll save you the WTH response if it doesn't make a difference....;)  The last could be cheaper and maybe more fun and educational. *S*
Thank you everyone for the great intell. I bought a Torus TOT MAX yesterday at a killer price and the dreaded hum is still there. I'm convinced its internal in the amp and something isn't right. This morning real early I powered it up with no input. Nothing else was on and I could hear a ver faint hum from 10 ft away.
I've sent an email to the local Anthem authorized service center, who told me this is normal, and we'll see what they say. As I mentioned before, there has never been a detectable noise (hum) from this unit until they disassembled and re assembled it. Worst case, I bite the bullet and ship it to Anthem factory at a hefty shipping cost.