What is making this unusual noise and where does it come from?

Check the shield around your catalytic converter it may have broken free...

Forgot to tell you car is in garage, not running, and no one in it. we open the door and bingo it goes off.

Not the catalytic converter. car not running when it starts making the noise when we open the door.

It's just saying hi to ya!

Thanks for writing. Sounds like what I found. I figured out that the outside air damper for the heater was snapping up and down. I reached in underneath the incoming air filter and pulled the damper out. The heater works mostly o:k but not quite right without it. BUT the disturbing noise is gone. - Sherwin