Weird sound from rear end passenger side when turned off

There is some sort of electrical box by the rear shock on the passenger side. Today while walking past my truck turned off I heard a sound coming from it and it sounded like a compressor running which is weird. Anyone know what that is

 

Like an electric parking brake?

 

Probably the key off EVAP test running like 2nd and 3rd gens, it's in your owners manual page 7.

 

Thank you

 

It’s the useless government mandated evap system waiting to throw an engine code after it develops a leak in a few years.

 

youre probably hearing the brake appying more force, were you on a hill?
mine did it the other day, truck was off for a few minutes & heard a few momentary noises, nothing to be concerned with...

 

Not useless to me. Keeps the garage from smelling like gasoline like it does with old cars. Fuck that, gimme evap cans