That's pretty wild!  I've experienced something similar - in Land Surveying we use magnetic locators a lot (e.g.  
http://www.schonstedt.com/index.cfm?page=GA-52Cx ) - kind of advanced metal detectors for ferrous metals - we use them to find old monumentation for parcels.
 
 When they are used over buried utility lines or under power lines you can hear a strange, creepy modulation much like in the youtube link - but the power line "hum" imposes itself over the standard detector beep/whine.  It also works on underground telephone and IT cables, but to a lesser extent - and mass transmission lines give the best (and creepiest) effect.
 
 Next time I hear it I'll try to get audio and post it.  Neat links, though!