banging noise

[lukbrw]

This is a goofy one. For the last couple of months, while driving we've been hearing this noise that sounds like a loud bang on a metal garbage can. The recently, when parked in the garage, out of the blue I will hear it again.

Did the same thing last night. Pulled the truck into the garage for a wash, about five minutes later, bang. Sounds ilke it's coming from behind the cab under the bed someplace. Gas tank??

It's a 2000 Silverado 1500. Z71. 180,000 miles.