That sounds exactly like the problem I had with my old 500. It was first during some test tones that I was doing when I first heard the rattle noise at higher volumes. Then I would also hear it during certain movie explosions or dynamic music passages.
Like you I snugged up the screws for the port and driver. I also checked to see that I didn't have any wires inside bouncing around on the port, since they kinda hang in there.
Then I discovered the rattle noise in back by the amp. I was easy to troubleshoot with the test tones as I could keep the rattle steady, then when I applied pressure with my fingers it would go away.
Good luck, and I know how frustrating this stuff can be.
