Vintage Synth Repair

I have a Hammond C-2 comming through a Leslie and lately its been 
making a noise that I can only describe by compairing it to the noise 
made by one clearing their throat. Something else that may be 
significant, the organ needs to be played for at least five minutes, 
before the noise occures. Also, I can volume down and noise will 
subside. I have sprayed the reastat, with no positive results. If 
someone has succussfully dealt with this or a similar problem, I will 
very much appreciate any tip that might help........ Thank you very 
much, Mark, from Missouri.............