That is exactly where your fuel pump is, behind the rear right "axle". In fact, you can just drop to your knees and see it. There ought to be both a fuel pump and a fuel accumulator there; propbably put down there to keep them cool and "easily" accessible. Buy you may also be experiencing an "echo" if you think it is on the left (driver's side), for there is nothing there except your exhaust. Could it be a loose exhaust?
Anyway, it seems that German fuel pumps start buzzing when they get "old", and they will continue doing this buzzing until they give up the ghost. They do not necessarily get louder with age, just start buzzing sometime after three-four years, and someday quit. That's why most people recommend that you replace them when they start to buzz - you'll get no warning when they are about to quit.
By the way, when they get really hot, such as with warm gasoline, like when you have little in the tank and it's hot outside, they will buzz even louder, until you cool them with a tank full of cold gasoline (like what comes out of tanks buried in the ground, which is 55 degrees F. year round, below 36" of ground cover).
So the choice is yours, replace now or wait until it stops - but given Murphy's Law, that might be in a thunderstorm late one night, 100 miles from home.....Andras