Bongo Fury

The window on my sliding door is making a rattling sound every time I go over anything remotely like a bump in the road (ie a glass against metal sort of sound). I've given it a good wobble to make sure it's not suddenly going to drop off and there's very little movement there.

Anybody else had this problem and dealt with it successfully (please don't tell me to keep it closed as it helps dramatically with the fresh air issue in the back, although I do appreciate that if I have the choice between losing the glass and keeping it shut, I'll go for the latter option).

Thanks in advance.

[Just edited this - it's making a rattly sound when it's OPEN]

Is there a rubber washer twix the bolt & glass?
If it's perished or missing that could cause a rattle...?
Is the bolt tight enough?
Again, the bolt might appear tight when the window is closed, But if it's unscrewed will become apparent when open...