Electrical buzzing noise near fuses in motor

I have a 1996 tacoma 2.7 that for some reason it has a consistent buzzing noise coming from the fuse box that's near the battery. I am looking for some info on this situation. My truck runs fine this has been going on for years and I am just getting more curious about it. Also I have had the truck parted for awhile and the battery disconnected and the buzzing still happens. When you open the hood all you hear is a buzzing near the battery and fuse box location. I appreciate any help if others have had this happen. Thanks

 

It's coming from the canister and vsv valves just behind the fuse box. Very strange sound. Equalizing tank pressures I believe.nothing to worry about .
Always heard it while working on them.

 

Yup, my 96 did it all that time.

 

My 96 does it too. Glad to finally find out what it was!

 

My 96 used to do it as soon as the truck started getting warm. It didn't have to be running. Put my 95 charcoal canister and vsv in place of 96. No more noise!! I always wondered if it had to do with check balls in canister or vsv.