Seems like a ground loop to me. Do you have a ground lifter to use for troubleshooting? http://www.amazon.com/45086-3-Prong-2-Prong-Electrical-Adapter/dp/B00166TT3G/ref=sr_1_3?ie=UTF8&qid=1395835417&sr=8-3&keywords=ground+lift+adapter
 
Rocky

Using ground lifters as a rule of thumb is not recommended. Unless you have a ground fault there is no hazard in using them to troubleshoot.
There are millions of homes in the U.S. wired with old 2-wire romex that have no grounding system without issue. Sure it's better to have a properly grounded system but using ground lifters to provide a ground may give you the false sense of security that you created a ground by connecting it to the device plate screw which may or may not provide a grounding means.
 
That 2-wire metal lamp sitting on the night stand wouldn't have a UL listing if this was so dangerous.
 
In my opinion there is no harm in using a ground lift to temporarily remove one of the paths to ground. This is common in equipment housed in a rack. The plug ground provides a path to ground but the metal case of the equipment that is screwed to the rack bonds the rack providing another path to ground for other equipment installed the rack which can create a ground loop. You can buy insulated washers for the 10-32 screws to prevent this from happening.
 
If a ground loop is the suspect the most effective way to troubleshoot is to remove a grounding path. I respect your opinion and I appreciate all of the information that you provide Bill but I disagree here.
 
Rocky

Lifting a ground using the adapters.... we've all done it on a gig at one time or another. No, it's never a good idea to defeat the grounding conductor. But hey, you're in the band, you've been hired to play music and the venue's wiring has ground loop problems..... fixing them is not your job or responsibility, and it's 10 minutes to show time......whatcha gonna do?   Yup, same thing I would do.... slap that ground adapter on the amp, and test to see if the mic shocks you.... if so, install a foam windscreen and be careful when singing.
 
In a studio that you will be controlling or owning..... different story. Fix the problem, don't just treat the symptoms.
 
I dare say that there are hundreds of studios, probably home and commercial, where the symptoms are treated and remain so.
 
When hum is caused by grounding issues, it does kinda cross into a safety issue as you start to work on the problem.
 
As far as the 2 prong or 3 prong cords.... all new 2 prong cords are polarized with the larger of the 2 lugs being the neutral.  And all receptacle outlets are polarized now as well.  I'm sure a few non-polarized lamp cords are still around but you'd be hard pressed to find them in a store now.
 
Most hand tools are now manufactured with rounded cords plus they use double insulation. So there are no parts that a user can touch that might be accidentally shorted hot.  
 
A few weeks back, my mother in law asked me and my son in law to look through the tools in her shed and pick out any that we might want. In that cabinet were several electric hand drills. the old kind with the metal casing and a 2 wire non-polarized cord..... Our parents used those old metal cased saws, drills, sanders, all the time.... There's no way I would have plugged those tools in and used them. We left them in that cabinet.

Okay, I could continue but I'm not here to change anyone's point of view. I'll agree to disagree.
 
Rocky