Basically the compressor is a piston/pistons going up and down on a crankshaft, same as an engine. Therefore it has bearings the same as a car engine. If they wear it will become noisy. The gas or lack of will not make it noisier or quieter generally speaking. It also has valves in it, generally metal flap valves, again these can wear and become noisy. The cycling will vary due to ambient temp changes, and also due to box temps. The fan on the condensing unit can also have worn bearings and can cause noise. You have to decide where the noise is coming from. The only definitive test that can be done on the unit is to attach a set of fridge gauges and a set of thermometers to the condenser and evaporator. Then the engineer can decide whether the unit is pumping efficiently or not. The units are sealed and the only way gas can get out is if the evap or condenser has been punctured (usually caused by a woman "defrosting" with sharp knife!)Gassing a unit is not a magic wand to cure all ills.The compressor of the Fridgomatic 12V (1995) fridge is very noisy when it "kicks in". The compressor runs 3 or 4 times an hour with the temperature control around half of of the scale. Not sure how often the compressor should run to keep the fridge cold but it does seem to be getting louder and running more often. Mainly used weekends and for summer holidays etc. Would re-gassing help? Also does anyone know of an engineer in the Chichester harbour area?