You answered your own question. Also, you may have an overfilled oil situation. Store your bike on a wheel chock that will keep it straight upright then you shouldn't have that oil pooling problem into the left cylinder.
I had the same problem. The Service Bulletin solved the noise. Call you dealer and ask the service dept. to look up.
BMW R18
PUMA - Service Bulletin
S1 13 07 21
Loud / Noisy Fuel Pump
Reply
Since it is an exposed system BMW recommends to clean it thoroughly and re lube the splines with TS-60 spline lube. A small stiff brush like an acid brush works well.
https://www.shopbmwmotorcycle.com/products/ts-moly-spline-lube-2-oz
There are probably many less expensive alternative gear...