I recently bought two beautiful straps from A Collected Man (ACM) for my CHronometre Bleu. I ended up returning them.
After I put the first strap on I noticed the rattling that some of you mention above. I took a closer look and noticed that the strap did not seem securely set in the lug holes. I twisted the strap gently and to my horror the strap came off. I immediately tested the other strap with the same result. I then switched back to the original FPJ strap and found that it did not rattle and it did not come off when twisting the strap.
My conclusion is that it is a combination of slightly too short springbars and some slight deviances in the design of the hole for the spring bar release knob.
I did not want to drop my watch while wearing it so I sent back the straps for an exchange. Still waiting. But your comments above make me wonder whether the ACM straps are actually safe in general.
Claus