All previous posters a/o Mapman 10/1/13 are correct. The arm has a ground fault in the left channel. Could be in the arm itself, could be the RCA cable, could be the headshell, could be the cartridge leads, could be all of those. Not worth fooling around with yourself.
What would cause...
a TT to play perfectly beautifully but, on occasion, emit a loud, constant "hum" through the left channel only? Trouble-shooting suggests that jiggling the tonearm cable lead coming out of the back bottom of the plinth can set off the hum and also disrupt the hum. This is a Rega model RP3 just purchased brand new. Should I insist upon a return or should I wait and see if the problem resolves?