Double Bass Schaller p/u picking up neck resonance?

[gitparker]

I rarely amp up - but when I need to the Schaller suits me
The open G string has a hum/ buzz - this is now sounding through the amp - from open G down to E on D string (decreasing) It sounds the same as feedback.
If I lean the headstock against the wall it disappears from the open G but is louder at E.
Got a gig tnite and haven't time to try isolating the p/u from the fingerboard - though I have it padded
Any ideas - not including scrapping anything!

 

[DoubleMIDI]

First thing is to find out if it comes from the bass or the pickup.
But before unmounting the pickup try rotating the magnets a bit. If one is loose it might buzz. If you cannot turn it, leave it that way because it cannot move and vibrate.
If that doesn't help unmount the pickup. If the bass still buzzes, it's the bass, if not check the pickup.

Your description is a bit strange because a buzz has a whole frequency spectrum, but a feedback usually contains mainly one frequency.