sniffing out ground buzz and hum

ran into the little odd ball thing today.  was over at a clients place, a sound stage.  Their mixer has slight buzz on it when  no audio is playing.  Makes sense to notice it when no audio is present. What is interesting is the mixer used an external power supply and when it was swapped with a known good supply. The buzz stayed the same and there was an increase in 60 cycle hum. Back on the original supply and we are back to buzz in the background that is noticeable.

I am convinced that their  building electrical is causing the problems except for the fact that the mixer unplugged from any audio connections minus a set of headphones, I can hear the buzz.
"
Odd one for sure.

A little troubleshooting 101,,,  Is the output hum independent of the console output fader? Does it respond to amp input trim?  Does it go away when mixer is powered off? Does it go away of feed from console is unplugged, but return when just the ground is connected?

Confirm that the audio path to the amp is clean...

Very rare, in some very strong magnetic fields, or from corrupted speaker wiring (grounds shared with outlet grounds) speakers can hum with no signal.

JR

all true. All things I am going over. We pulled the mixer and brought it back to the shop for further testing and evaluation...

traced it down to the power supply. Replaced a couple of failed tip 141's and no more hum....

One of my "go to" tools recently has been a 1/4 guitar cable that I stuck an alligator clip for ground and a big cap with a heat wrapped wire prob on the other line that I plug the other end into my amp and prob the circuit with a test tone going through it...helped me find a missing cap on my GSSL noisy channel and also the failed BC550 on my Green pre...it's one of the simplest and fastest ways for me to trouble shoot circuits these days...

yup I have an audio sniffer. 
Yes I have an audio sniffer I use at times.