A very brief burp/pop sound emits from the power tube every time the cocking rod resets the bolt. The gun shoots and functions just fine aside from the burping sound, but it's starting to get on my nerves as it takes away from the much more satisfying "click" I used to get.

The powertube assembly is level 7. It has the stock foamy-less bolt and solid brass spacer instead of the spring spacer. I tried swapping out the entire powertube assembly, including the bolt and spring, from two other level 7 guns, both of which had the spring spacer. On both attempts, the problem worsened significantly, with a much louder "psst" sound and noticeable air movement coming out the barrel upon each time the gun was cocked. The original assembly was therefore re-installed.

Next, I tried swapping out the standard on/off assembly with an RT on/off. This made no difference. I'm stumped at this point.

The sear is in good shape. The trigger rod is properly adjusted. The gun is oiled. I'm using a full tank of compressed air. The wave spring is still fresh. Aside from the pump milling, everything on this gun is stock MM parts. I'm stumped.