My driver's side door has always been pretty darn solid, even with the window down, but my passenger's door was always horrible, especially with the window down.

I found out why when I finally took the door apart to upgrade the speakers - my passenger's door was missing the sound insulation and a nut from the window linkage had backed off substantially.

I tightened and fixed the linkage (and adjusted the window) using onehundred80's great guide (search and you'll find it pretty fast).

Then I stuck some Dynamat to the plastic backing (which was mysteriously still there sans insulation) and put the door back together.

All is well now and it sounds firm and solid.

Taking the door apart was fiddly and annoying (lots of clips and iffy prying, especially on the silver/chrome handle covers) but quick. Took me under <30min per door, and that was including speaker disassembly and reassembly. If your doors make noise, might as well open them up and see what you find.