I have an 87 740 Turbo Volvo in addition to the Vanagons in my
driveway. As you can imagine there are listserves for Volvos as well.
This symptom of a buzzing fuel pump is a pretty common topic on the
Volvos. The stock fuel pump on these VOlvos is also the mighty Bosch.
The explanation for this buzzing is a faulty fuel pressure regulator,
which is after the fuel pump and located on the top of the engine near
the fuel injector rail. It is is that brass-colored thingie on the top
of the 1.9 and 2.1 L VW engines. The buzzing is due to the fuel pump
being run all the time because the regulator has failed. Running the
pump all the time is bad and will shorten its life.  The regulator
pretty cheap part, $30-40.