Hum in Cary cad 805 AE

Hi Dear fellows.
I recently noticed a hum in one channel of my Cary Cad 805’s.The Hum is present once I switch on the left monoblock.It is not heard through the speakers, only in the amp itself.What could it be the problem??I calibrated the bias of both amps, exchanged power cords, even the 211 tubes but nothing changed...there is still this hum in the left channel.Any recommendations will be more than helpful!

Thanks in advance!
Raf