Mine USED to do it once per day... this last trip, it started doing it every time the **** compressor cycled on and off. Sounds like it could be a plastic door somewhere on the firewall opening or closing, but it strangely did it everytime the engine noise changed with the compressor cycling on. Let me tell you, after 4 days on the road, it got really annoying! It may be "normal" but it's not acceptable for a $40K piece of engineering!

 

Mine does it, FOOM......first start of the day...everyday. I don't like it, but have gotten used to it.