My fuel pump started buzzing some days ago so I replaced the pump and
installed a second fuel filter between it and the tank, as a pre-filter.
The buzzing went away for a few days but reappeared.  So I removed the pump
again and attempted to clean it by adding input and output hoses, placing
them in a pail of gas and running the pump.  I also replaced the pre-filter
and noticed that there were rust particles in its inlet pipe.

Again the buzzing went away and then returned after a few days.  I'm
guessing this the pump is cavitating.  I noticed yesterday that immediately
after I refilled the fuel tank, the buzzing was quite diminished though
still audible.

I'm thinking that the filter in the sump of the tank is clogged.  Could it
be rusty?  The PO had parked this '87 Westy for a few years, following an
auto transmission failure.  If that is the problem, what is the solution?
Or is the cause of the buzzing noise possibly something else?

Thanks, Rick