Great post Bill well explained, thank you.  I think for troubleshooting purposes sometimes lifting a ground temporarily can be of help as long as its very temporary only.
 
Another thing Firebird mentioned was the Humfrees for isolating rack gear and these are great.  I had a reverb unit that always had a very minor hum issue, not bad but annoying.  Isolating off the rack solved it very nicely.  I only needed to do this for one piece of rack gear.  If you suspect something like this try pulling it out and see. 
 
Another thing that can cause trouble too is where some of those in line power supplies can be placed. I had another effects unit in my rack that had a slight hum in the audio as well.  I spent years on and off trying to track this one down and one day I accidentally kicked a power supply on the ground behind the rack and it turned 90 degrees and the hum went away!  I could not believe it.  I just never thought.  Obviously a hum induction issue but once I relocated a few of them into a far away corner somewhere quite a few hum issues went away after that.  (even if it means extending their DC cords a little) Be aware of those things, they can radiate quite badly and fool you. Be prepared to turn them at different angles etc.. A lot of our gear has them now which can be a bit annoying from this point of view.
 
With care you can have total silence, hum and noise wise in your studio. 

fireberd
I used the 2nd Class Radiotelphone when I worked for a Motorola 2-way radio dealer (like in police, fire vehicles, etc).  The 2nd class was required. Capital Radio Communications in Lemoyne, Pa (Harrisburg suburb).

Lemoyne? Small world, I used to live in Womelsdorf, and dealt with Capital Radio when I worked at Met-Ed. Never worked on commercial two-way gear much, but did have a bit of test equipment envy when I visited our local two-way shop. Pretty cool stuff!

I can't remember their name, but they were great. I had a deal with them where they would monitor the college FM station where I worked for center frequency and modulation a couple times a month as a backup for our modulation monitors. The rules were somewhat relaxed for low power stations at the time, but it seemed like a good way to spot problems. I think they charged me $15/month for the service.

Ah the good old days<G>!