Korg Poly800/EX800 Users

I've got a problem with my synth. Listening through an AMP, 
headphones or even recording. Whenever I play any key on my keyboard 
I get a hissing/popping/static type sound.. it's not to loud, but 
enough to where it causes anything I record/do sound bad. Has anyone 
ever encountered this problem or know what to do? I checked all my 
grounds. Everything seems to be good. It does it with just batteries 
or with the power supply. I opened it up and there was some gum type 
substance over between the C3 and CN9A area. I removed it, was I 
supposed to do this? I tried adjusting the course and fine tuning. 
did not do a thing. Adjusted all my grounds. This sounds barbaric, 
but if I just smack the synth it will go away for an instant. but 
come back.. I think the longest it went without this problem was for 
5 minutes. If anyone can help me with this problem I would really 
appreciate it. Or at least guide me in some direction.