"Ignoramus24985" wrote in
message ...

This valve opening produces a very annoying noisy blast of air.

i

Perhaps a pipe-threaded lawnmower muffler would help?

I found a bin of air exhaust silencers similar to this in a
second-hand store and use one on my vacuum pump:
http://www.amazon.com/Amico-Plastic-.../dp/B00A7629II
The porous material inside resembles that used on chainsaw intake
filters. The thread is more likely 1/4" NPT than 4".

I just tried it. While it doesn't "silence" the loud pump outlet it's
quite effective for the price. It drops the sound level from 92 to
82dB and removes the annoying buzz from the vanes.
jsw