I have an Advanced 40A (about 6mo old).
Sometimes when the relay engages I hear (and feel on the case) a loud 60hz buzzing. It happens immediately when the relay engages (not based on load of the car), and continues for the entirety until the relay disengages.
The weird part is sometimes it buzzes and sometimes it does not, I haven't noticed any reason why sometime it's quiet and other times it buzzes.
It's quite loud (hearable from 50-100ft away)
The relays with AC coils do buzz at 60Hz at various intensities. There is likely nothing wrong. If you would like to try another we can send one out.