For the life of me, I cannot make any headway on this rattle i have. It's a high frequency one that is definitely between the door skin and door crash bar. I have fished things behind it, blown air around, tried to look in there...nothing.

I am considering low/no-bow expanding foam to calm down whatever is in there. Would that pose any inherent increase in rust creation?

I'm sure it will to some point but if the rattle is driving you crazy just do it...Too bad you can't see what's going on.. I'm thinking something is lodged between them..I've seen where the crash bar somehow gets broken welds and does what you're describing also...Bottom line is the easy way out is foam it just don't go overboard...

Did you check that your door lock rods have the protective sleeves on them so they don't rattle when they touch.

This ^^

Most common cause of door rattles.

Yes, definitely not the lock rods.

I would use seam sealer before that foam. You know it's not going to cause rust and if the parts are close enough to rattle, the sealer will stabilize them.

Agree, seam sealer or that spray in lizard skin sound control. When I called 3m about this exact problem they said to use nvh 4274 between panels, it is sound deadener, vibration eliminator and is designed to go between panels and not print thru, or superglue it like fusor, also the best for between panels like your hood skin. 3m said other products may work, but nvh is designed for this. The question is can you get it where you need it with a caulk tube or not, if not look into lizard skin or other car stereo sound spray.

TURN UP THE RADIO!

open exhaust werks 2

Before using the seam sealer why not try stuffing something removable in that space(s?) - a piece of foam rubber, or just rubber, even paper or something just to diagnose and confirm where the rattle is coming from. If it isn't where you think it is you could go through a bunch of seam sealer and not fix it.