The Progressive Dynamics Transfer Switch is "Generator Dominant" which means that the contactors are operated only when shore power is being fed to the RV Breaker Panel. This is why you get "hum" when plugged into Shore Power.
This Transfer Switch is compatible with Generator Auto Start. If your RV does not have Generator Auto-Start then one possible solution would be to change out the Transfer Switch with a different brand that is "Shore Power Dominant". One example would be the WFCO T-57 , (if your RV is 50 Amp Service). Another advantage of this one is that WFCO manual says it can be mounted in any orientation. You might still hear hum when the Transfer Switch Contactors are operating, but it will only be with the generator running. If you can hear it over the generator running noise, I would be surprised.
Randy - Manhattan, Kansas
2015 Vista 27N
2016 Honda HR-V