When you say the TT is not on I'm assuming you mean that it is connected to power, but not in use? If so, is there a third lead from the turntable, in addition to the left & right channels?

My own TT has such a lead which connects to a dedicated earth terminal on the amp. On one of my amps disconnection makes no difference, but on the other one disconnecting produces a hum, while a loose connection gives hum + crackle.

Of course if there's no 3rd lead, or no earth terminal, this is no help, in which case sorry, no idea!