what are these two rubber circles under the hood they keep humming??

everytime i start my truck then shut it off these two rubber circles start humming they hum once every 40 seconds or so sounds like something electrical to me just not to sure. any ideas what this could be. it does it for about a hr or so after the truck has been shut off.

 

yea thats it its right behind the fuse box

 

yup you where right i got my lazy but up on the internet and did some research lol its called a EVAP charcoal canister guess the 96-97 tacomas did this its called the bee buzz sound or something like that. again thanks for your help.