Question; Did the tech at G&L acknowledge hearing the same noise you're hearing?
If yes, but they couldn't "pinpoint the problem", then they should have gutted the elecs, and installed a whole new harness. Everything, from the jack to the pickup, and not send it back to you until it's fixed.
There just aren't that many elec components in a P bass that can go bad. Even if the problem is
intermittent, a total elecs swap could rule that out.
I'm reminded of a CS experience I had with a brand new BMW K100 motorcycle. One cylinder would quit firing after a couple hundred miles. I'd take it back to the dealer, they'd "fix" it. Couple hundred miles later, same thing.
The third time I brought it back to the dealer, the head BMW Service honcho from Germany happened to be there. He came into the waiting room where I was and asked "Is that your K100? What's wrong with it?"
I explained the issue to him. He apologized to me for the inconvenience, then summoned the owner of the dealership to the waiting room.
He told the dealer to:
1) Replace the entire fuel injection system.
2) If that didn't solve the problem, he was instructed to replace the entire engine from another new bike off the sales floor.
3) If that wasn't satisfactory, the dealer was to simply give me a whole new motorcycle.
I was blown away by BMW's "spare no expense" approach to CS, and have been a BMW rider ever since.
Maybe I'm spoiled, but something as trivial as a faulty bass guitar? I'd feel 'punked' if I got the run around that you seem to be getting from G&L. C'mon G&L...really?