I have a small heat-pump on test, in my lab. The unit has a capillary tube installed, together with inlet filter/strainer.

At various points during the heat-up phase, noises like machine-gun fire, can be heard - from time to time. I traced the main noise to the junction between capillary tube exit, & evaporator inlet. Checking the sight glass shows a fair amount of small bubbles coming up from the condenser coil.

At times, the system goes completely quiet again. The noise seems to be associated with the bubbles, as is to be expected.

This is the first time I've worked with a capillary tube expansion device, on a heat-pump.

Does anyone have any thoughts on how to reduce the expansion noise?