I have heard and tried the following: Remove the PG and wipe it down with a clothes dryer sheet (bounce,etc.). Hope that helps.
I've heard of this happening before, but never to a MM bass. It is static and it is the pickguard. Why? I don't know, but I know how to fix it if the new PG does the same thing. I've heard about the clothes dryer sheets, too, but you can also put some shielding foil on the back of the pickguard and that will kill the static.
Just make sure that the foil doesn't contact metal (pickguard screws, pickup covers, etc).
I had some instructions around here somewhere...here we go:
"...shielding the back of the pickguard can help wick away the static buildup that sometimes causes popping noises while you are playing.
Lay the pickguard face down on clean newsprint or something else that you don't mind getting sticky. Spray a light coat of trim adhesive on the back of the pickguard. Follow the instructions on the label. Be careful to spray straight down so that you do not get adhesive on the front of the pickguard around the edges. When the adhesive becomes tacky, move the pickguard to another clean surface and lay a single piece of foil on the back of the pickguard. Working from the center outwards, press the foil against the adhesive. Carefully smooth out any bubbles or wrinkles.
Trim the foil from around the edges. You can either trim it right to the edge or you can trim it in about 1/16". If you trim it to 1/16", rub the adhesive off with your fingers so that the pickguard won't stick tightly to the body when the pickguard is reinstalled.
The only critical area is around the pickup slot and holes for the pickup screws. Here you want to make certain that you remove enough material that the metal cover on the pickup and metal springs on the pickup mounting screws don't contact the foil. The pickup cover is grounded and if it also contacts the foil on the back of the pickguard (which will be grounded through the cavity shielding) this will create a ground loop that is susceptible to noise."
Haven't had to do this myself, but it makes sense.