Is this normal

I just aquired my 96 Tacoma, and have asked previous questions on this site before. Well, now I have another. When the truck is just sitting, I hear what I'll call a"VEERP" noise. At first it sounded like the air conditioning system equalizing, but it's comming from the charcoal fuel vapor canister. It does it all the time! The truck can sit for a week without running and every few minutes you hear this "VEERP" sound comming from under the hood. Is this normal or should I be concerned. If anybody can fill me in on what is causing this, I'd be appreciative. Thanks
Tom

My 92 did the same thing with the A/c on almost from day one. I was told it was the expansion valve in the A/c system. (probably had a little trash in it)I drove it for 228K miles and never had it fixed. It worked fine the whole time. If you can stand the "hungry baby bird" under the dash I would just leave it alone. Don't fix it if it isn't broke.