Hotpoint RLA36 Compressor Suddenly Noisy & Hot

This is a four year old Hotpoint RLA36G which has always run quietly until about a week ago. The fridge is still cooling properly, but the compressor (and it is definitely internal to the compressor) is making a loud-ish buzzing noise. I measured the sound level and its maybe 50dB or more, depending on the distance to the sound meter. I also noticed that the compressor is very hot to the touch – A point-and-shoot thermometer reads 70degC on the compressor casing. Cooling coils on the back are hot for the top 6 inches but cool on the lower part. I’ve taken the PTC off and checked resistances and both it and the compressor motor windings measure ok electrically. My impression is that the fridge is now cycling on for much longer than it used to, but can’t be sure as it was always quiet before. Am I looking at a compressor failure or coolant leakage, and can the coolant be recharged on this appliance, or do I go for a new fridge? Any advice gratefully received. Thanks. //Jeremy