I noticed a very slight hum coming from my 30 amp hardwired EMS and was told by Progressive Dynamics that a very slight hum is normal; it originates from the relay that is held open when power is passed through the EMS. I mounted mine on a piece of foam and put a piece of foam on the lid and the hum could no longer be easily heard.
This was a very small, very slight hum.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch