I have an Epiphone Les Paul with P90s in it, and I haven't played it in a long time, but when I plugged it in this week, it is really Humming. This is not just when I am not playing but while I am playing, in that it is almost as loud as the notes I am playing.

When I go to palm mute it really gets funky sounding, not sure how to describe it, but it doesn't sound like a palm mute.

I have also turned of monitors and lights and it still happens. I figure it is some sort of grounding issue (especially if it goes wacky when I touch a string) but I am unsure of where I should be looking to fix this. Also it is not the cable, as I have checked with 2 other guitars (both humbuckers though) and they show no signs of hum even with lights on.

Any help of where to start would be appreciated!
Try a different amp to rule that out.

Open it up and clean all the electronics. If that doesn't do it, it could be a grounding problem but it's hard to say without hearing it. Also, P90's do hum, especially with a bit of gain.
There's no point which is more common to develop ground issues than any other, although in some older Epiphones the toggle switches were slightly more prone to problems than other parts. All you can do is open up the back of the guitar, check to see if anything looks like it's come loose or if any of the soldering looks like a cold joint (dull, not shiny).
Yes, I know everything. No, I can't play worth a damn.
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Ok thanks, it shouldn't be the amp, as I am using an amp sim, so that should rule that out! But yes, I was hoping there was a common wire I could check, but I guess at some point I will check the whole thing over, maybe even re-solder the conenctions, as I had done some work on it when I didn't really know how to solder, but it had worked at the time. Now I'm a bit better at soldering and understand cold solders lol.

And I know P90s hum, but this is like 10 times worse than any single coil I have heard, to the point that you can't even play the guitar, plus it hums bad even on a clean channel.

Thanks for the help.
Sounds like the guitar needs to ground wire to be reattached is all.