Just thinking out loud here.
Another possibility is a water plug at a low point in the air lines. Air pressure would build behind the plug until it finds its way past at which point your diffuser would burp. Then the water flows back to the low point reforming the plug until the air pressure builds enough to find its way past again then another burp.
It only takes 1 psi to raise the theoretical plug 27" so your gage may not show the small change in pressure.
