I started using a compressed air dryer. This dryer, as part of the
operation, opens a valve every minute or so for a couple of seconds.

(it is my strong opinion that I should try to find a way to make it do
so less often, but I digress).

This valve opening produces a very annoying noisy blast of air. To
combat this, I routed the polyethylene tubing to the outside of the
building, by drilling a hole in the masonry.

This "resolved" the problem by moving the blast from the inside to the
outside. The outside is the back of the building and faces the
railroad. So it is not bad. But, I would like those blasts to be
quieter on the outside also. What can I do, realistically, to silence
them?

i