Thanks! Yes that GE is nice and quiet. That is what GE had, in competition to the belt-drives of the time. No wonder their idea of a hermetic motor is the one which won out and we still use today! That one was found rusting in a junkyard, seized up. It was an interesting road to recovery for it.
The Norge does chug in an interesting manner. Its compressor runs at a fairly high speed as far as belt-drives go. As with all of them just being started, the load on the compressor was highest with the evaporator hot. That makes the single compression stroke of the Rollator seem more pronounced, because of the higher load on the unit. When it's cooled down and cycling normally, it doesn't chug like that.
The Frigidaire is a twin cylinder pump at lower RPM. Again, the compressor makes that "tinking" sound when it's under a high load at startup. I think it's coming from the compressor valves. The inlet valve is in the piston, and as the pistons reciprocate, the valve disc rattles up and down. When there's less load on the unit, you don't really hear that.
They all do have their own sounds. That's what I love about working with and collecting all the different old machines like this!