Anyone Know Where I should Look to Figure Out This Hum?


http://https//www.dropbox.com/s/faxmop972rok7i1/Noise.m4a?dl=0

I recently purchased a Lamm LP2. First couple days, silent. Now this. It's coming from both sides equally, turning up the volume makes it louder. It's not the cartridge. Noise still there when I unplug the turntable inputs from the phono preamp. It sure doesn't sound like a ground hum and since it's both channels I don't think it's going to be a bad tube . I have another set of tubes on the way but they won't be here until next week so I can't double check that. I swapped out the rectifier tube and that changed nothing. 

Help!
dhcod
Thanks so much for this confirmation. Now I just have to figure out how to get it repaired without shipping cross country. Can't find a schematic online so there may be no other option unless Lamm will provide one. Hopefully they'll respond soon.
Post removed 
I was wrong. It was a ground loop which was solved easily by the first thing I should have tried which was putting in a 2-prong adapter on the Lamm. Done. Noise gone. 
Curious that Ralph said it wasn't a ground issue. While it's common enough to hear 60/120Hz buzzing, I have heard a heavy hummmmm when a lead pops off a cartridge.