I Recently got a RC-120. It makes a slightly annoying hum. some people have said this is normal, others say grounding issues. its tolerable but I want to make sure its not an issue. any thoughts?
On the JC120's i've played i've always heard a slight hum. It usually is 60hz hum from the electrical supply.

If it gets "unbearble" then it might be a grounding issue on the guitar
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first you should eliminate other possible outside issues.

move the amp to a different location.
use a different cable, no pedals,
a different guitar.

see if anything changes the hum volume.

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My JC120 head just had that problem but then i realized that the dude that had it before me pulled the ground prong of of the power supply.

Check that. But if you have one of the newer combos and you've tried everything, the new combos have an fx loop. I'd toss a noise suppressor of some kind in there.