Low humming noise out of prime sitting in my garage (not charging)

yes, just an evap check. scared the dickens out of me the first time i was in the garage when it started

 

That's a frequently-asked question around here. You can read about it on page 8 of your owners' manual:




For the morbidly curious, another of the many threads where it's been discussed has a diagram of exactly what the car is doing in that test. This picture is from the Gen 3 repair manual, but the Prime details are probably not a whole lot different.




You can see you might hear that pump running for as long as 22 minutes and change, depending on how long step C goes.