most common form of what most call as a ground loop is when we hear the nice 60hz hum in a bands loud speakers, but also occurs in video equipment, computer equipment (older network stuff) and a few other things.
the most common cause is because of neutral current on the equipment grounding conductors between two points.
Here is one example:
a band has set up there PA system and amps on a stage in one corner of a club, they install there mixing board across the room, the stage has a panel on it, but is only wired with three conductors (PVC pipe) neutrals and grounding is bonded together at this stage panel, the mixer is on a receptacle from another panel, because the neutral to the stage is also the grounding conductor the amps, are grounded to this current carrying conductor, so is the snake shielding, and since the pathway has current on the grounding, and current takes all paths, some of this current will also be on the snake shielding, which the amplifiers will love to make louder.
this will be also the same setup where audio or video cables run between buildings that have differant services.
the other is just where a neutral has been grounded some where in the grounding path between the equipment, intentional or not?
for almost all audio equipment you can just put in 1:1 isolating transformers for the impedance your using I.E. XLR to XLR, ?" to ?" or even RCA to RCA, When I did sound I always kept these in my bag of tricks, and the same for video, but for the older computer networks see Gars above post, PM him