I've just started seeing a small problem with my 300D. I'm wondering if it isn't telling me that its days are numbered. I've got about 15k actuations on it.
When I first touch the shutter, I get a quiet little click and a flicker in the viewfinder. The shutter fires normally when fully pressed.
No IS lens.
I know I noticed it with my Sigma 150mm macro on...I can't say for sure if it has done it with any other lens since I shot with that one lens most of the day.
I'll have to try things out.
Diaphram...that sounds like a real possibility.