Everyone seems ready to jump all over the seller without giving things a good look. I would think the first thing to check would be the RCA jack at the pre-amp. Wiggle the plug and see if that makes a difference and also try coating the plug with De-Oxit. Problem may also be a loose pin in the cartridge. Try it in another tonearm. Trouble may well be in the tonearm wiring but I'd check the simple and obvious first.
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?