I tried  running it with the gas cap off--no change. Also took apart the
cannister  but didn't see anything abnormal. What part do you suck on to
check? And  how do you keep that nasty stuff out of your  mouth?

hehe,
Well, I was quoting theory not practice about the sucking part .
I simply meant it needed to be vented which it sounds like that's not the
problem if it doesn't change with the cap off.

I would try blowing (with compressed air) back into the tank at the line to
the square filter.
(Take the cap off first)

If that's good I would change the high pressure filter again or at least  try
blowing through it to check for flow.

If that's good , I would run a line from the pump outlet into a big gas can
and check the "unloaded" or no pressure sound of the pump.
Still noisy?  maybe a pump ....LOL

jeff

87' Westy "weekender"