Not a Halloween story...

I was out in the garage about 5 minutes ago when I hear this weird mechanical sound. At first I think it is the AC unit for the house but when I figured out it was not that I started eliminating possibilities. Secondary AC unit, water heater, underground sprinklers. Only thing left was the Jeep and the Tacoma. I circled the Tacoma and narrowed it down to the passenger side rear quarter panel. It continued for two minutes, almost a purging sound. I drove the Tacoma this morning but it's been sitting in the garage for at least 4 hours. Any ideas?

 

The fuel system runs a check ever so often when the truck is off, I would bet thats the source!