If you are certain that the doors are secure then wait for a warm day when your truck has been i the sun for a while or get a blow dryer/heat gun and here is why.
If you look at the top right corner of back door interior you should see a little foam pad, about 3/4 of an inch by 1/2 inch. The top will likely be flat and the bottom will be puffed out. this is glued on and where the heat comes into play. move the foam pad up anout 1/4 of an inch.
repeat every few months as the noise returns. Or get a larger piece of foam.
1996 Supercab Splash 4x4. Man. - D44 front axle - R7 rear axle - Rusty's Xj Long arm upgrade - ProComp 8179 15x8 - 33x10.5 BFG KM2 - Bilstein 5150 - Skyjacker FR36 - 5% rear tint 50% front - Optima Red top