About Jack Reed's reply: it might really just be bad gasoline, in
particular gasoline tainted with water. The pump on the '75 made the same
howling/humming noise and we found water in the filter (and dirt, too). A
mechanic explained that because water is heavier, it puts the pump under
strain. Moreover, water doesn't lubricate as well as gasoline. Hence the
noise. Could be bull, but it made sense to me at the time. Note that water
would also cause the engine to sputter. When you remove the fuel filter,
hold both ends shut with your thumb and index and shake it up. Then dump
the contents onto a smooth surface (like into a bucket). If you see
"pearls", then you got water. 

-- 
Sami ('75 Westf.)
dakhlia@wuecona.wustl.edu