I had this problem, and still do.
You want to know the answer?
I am 150% sure that the outlet you are playing on is not ground. I might be a 3 prong outlet, but the electricians that installed it never grounded the damn thing. If they did, the guitar would be ground 24/7.
There is nothing wrong with your guitar.
I know this because I took off the wall outlets faceplate and looked inside. then I plugged it into the 3 pronger in the the kitchen and it went away. The kitchen one when I took it off was grouned. Then I plugged into the bathroom outlet and it buzzed again. Took off the faceplate and what do you know....no ground wire.