Dashboard rattle

Ever since I had the heater and AC cores replaced a few years ago I've had this rattle noise in my dashboard somewhere in the right side near the right side defrost vent. Maybe behind the firewall where the AC plumbing goes in. I have had the glove box out and can not see any thing that would make the noise as the big round housing for the fan is blocking what ever is making the noise. This noise is more noticeable on rough/gravel roads but sometimes you can kinda hear it with the truck stationary in idle. Noise can not be heard leaning down below top of the dash level. I have used tissue to plug the right side defrost vent but sound does not change. Sounds like an empty water bottle is loose in my dash somewhere.

Anyone else experience this?

 

I had something similar that, after some digging through this forum, sounded most like a broken clip on the dash. I took some expanding weatherstripping and packed it as far down the windshield as I could near where the sound was coming from to compress the dash as though the clip weren't broken. Haven't heard a peep since.

 

What is "expanding weatherstripping"? More specifically, what did you use to fix your rattle? Brand? Size?
TIA

 

I used something

$15 Foam Strips with Adhesive, 1" Wide X 1" Thick, Neoprene Weather Stripping High Density Closed Cell Foam Tape Seal for Doors and Windows Insulation, Total 13 Feet Long(6.5ft x 2 Rolls)

like this (don't remember the exact brand, but this is about right.) Cut little 6-inch strips of it, used a dowel to push them farther down the dash than my fingers could go, then kept adjusting/adding until the sound stopped.

 

Thank you, I appreciate your reply!