finally found the LINK!
https://www.ford-trucks.com/forums/1...sible-fix.html

 

Not that it matters when we're talking about door noise, but I have a 2002 Ford F-250 4X4 V10 extended cab, with a rattle noise from the driver side rear door. Talking about driving me nuts for months! Door panel off, couldn't see anything wrong. Pinch weld intact. Tried everything.
Finally, I tried driving with driver's FRONT door ajar (but with safely catch still holding door from opening completely) -- noise same or worse.
Tried driving (slowly) with door open (completely unlatched) -- noise gone!
So I lubricated the FRONT door latching mechanism that holds onto the extended cab door -- noise gone.
I seems, in my particular case, the rattle came from the front door latch mechanism, but resonated into the rear door area. There are probably different causes for the "same" noise.

 

Door seal mod and check the welds.
this is a VERY common issue.

 

There is a thread with pics of what I am going to describe..

Look near the bottom of the extended door, there is a 'screw/rivet' which holds the rubber gasket/inside edge of the rear door "snug" with the door. In my '99 F350, this rivet/screw is broken off, and I can move about 1/4" the gasket, so when I close the main door it doesn't git tight and the inside door panel of the rear door rattles a bit. There is a fix either find a small screw, or drill for a new screw, to replace the broken screw/rivet and it'll tighten up that door panel.

AHA, SuperDutyScaler found the link already...