I believe the Progressive PD-52 contactors sit in a neutral or non-connected state until power is sensed on L1, either from shore power or from the generator. The transfer switch "clunks" when power from either source is applied.
The PD52 prioritizes the generator. That is, if shore power is present and you start the generator, the transfer switch will wait about 30 seconds and then change over to generator power. If you start the generator because of a power outage, when shore power comes back on, the transfer switch doesn't automatically switch back to shore power - you must stop the generator.
Your ESCO switch has a normally closed position of contacts for shore power. So there's no "clunk" or activation when on shore power only. But I believe if you start the generator while on shore power, after a delay, the switch will change over to the generator automatically. That's what's called generator priority.
As for you buzzing noise, most transfer switches do not make a noise other than when the contactors fire. There was a malfunction on your PD52. If it started doing that while under warranty, Heartland or Progressive would probably have sent a replacement unit or authorized warranty repair.
Is the normally closed shore power design better? As with most design choices, there are likely pros and cons. For those of us who rarely use the generator, a normally closed shore power connection would probably prolong the life of the transfer switch.
