What makes an air system howl?

[D60]

So I recently plumbed air into the shop after moving the compressor outside. Using 1/2" brass except where noted:

enters shop, then thru coalescing filter (3/8" fittings), then to one unregulated output and gauge for high side, then to regulator (again, 3/8 fittings), then to my manifold.

The thing is I get this annoying howling sound sometimes, like what you'd expect when an indoor plumbing system has "air in the pipes."

It seems more inclined to make the noise under heavy load, but will sometimes do it with every little tap of the trigger of [insert air tool here]. It also seems more inclined to do it when it's hot outside, as the temp drops in the evening it seems to occur less, but not always.

Basically, nothing is entirely consistent so it's hard to diagnose. I don't know if this is a volume issue with my two reductions to 3/8", or a temperature issue, or both. Perhaps hot air from the tank is being cooled in the pipes?

Finally, last nite my CH regulator started blowing off pressure for no apparent reason. I got it to stop by spinning the impact for a couple seconds (adjusting the knob on the regulator did nothing). Ever since then, no more noise, so I'm wondering if the regulator was howling. Has anyone ever had a regulator make noise?

Has anyone ever had a compressed air system howl?