How to see if there's DC at my AC outlets?

"1ST Setting your voltmeter to DC and plugging it into and AC outlet will probably smoke the unit."

There will be no smoke, that will be fine if you want to try it.

"2ND Direct Current is not ever present in Alternating Current line nor visa versa."

Sorry, that's not true.

The buzz may be caused by noise on the line. A common culprit is a dimmer switch. I used to have an isolation transformer that buzzed loudly when the light in my stove hood was dimmed.

What if I test the hot to ground or neutral to ground?

I tried isolating just the amp, with nothing else plugged in, and the variac still hums / buzzes.

PS Audio has a product called the Humbuster that stops DC from getting to your electronics which causes the transformers to hum. So how does the DC get into the lines, then?

Did you try going around your house and turning everything off to see if it stops? If you are in an apartment it could be your neighbors stuff.

Does the variac hum with nothing plugged into it? If it only hums with something plugged into it then something might be loose in the Variac, but I still think noise on the line is the most likely culprit.

What's with the Variac? In my experience, all Variacs buzz, but some buzz more than others.

El, I have a Variac that has never buzzed that I noticed, but it is rated at 15 amps and I probably never got close to that limit.

Mark, now that I think about it, why are you using a Variac?