I'm new to this site, but have been RVing for 20 years. I do all my own maintenance as I am an ASE certified mechanic and a licensed general contractor.
If you have a unit with a fan, you are probably hearing the fan switch on and off. The cooling unit is almost silent except for a small burbling noise from the boiler.
I had the same problem with my Dometic 2652 and determined that the thermodisc switch was located incorrectly. It was directly in the flow of the fan and when the fan kicked on it cooled the thermodisc and shut the fan off in a couple of minutes!
On-off-on-off... It drove me crazy.
It's an easy fix, but you have to pull the refer. Move the thermodisc all the way to the right (facing the back of the refer) to the bend as the tubing goes from vertical to horizontal. This is the hotter part of the tubing as it is before the cooling fins. It's just a snap on switch, so pull it off and snap it in the new location.
It takes about 30 to 45 minutes to pull, move and reinstall. Don't forget to soap test the gas line after reconnection.