Help diagnosing a loose window

[frankienap]

I've searched quite a bit online (including this forum) but can't seem to find any specific fix.

Issue - whenever my passenger-side window is partially down, I experience rattling. This can be when driving over a big bump but especially whenever a passenger closes the door (in fact the rattling scares me so much I'm afraid the window is going to break into a million pieces). Note - I don't experience any of these symptoms when the window is in the fully upright position. (Which makes the think that perhaps it's not the window rubber sealer?).

I would like to try to fix this myself is possible. I believe the first step should be to take off the door panel (?), but after that I'm not sure how to diagnose. Is it the window assembly? Possibly a loose bolt or bolts?

Appreciate any advice. I live in San Diego and I like keeping my windows down. But I'm scared to death I'm going to shatter it if I leave it down by mistake before shutting the door.