When it is raining, a barrel with the lid off makes a "open/close lid" sound effect about every 10-15 seconds.
While it's only moderately irritating, it does give away the location of otherwise hidden bases/stashes. I found a few bases I would have otherwise just walked past if not for this bug on the exp branch, one was on a rooftop, the other fairly well hidden in a group of trees.
Here is a 47 second video of the bug. (m4v version)
in case that link won't play (webm version)
I did notice one interesting thing about this bug. When I moved my testing server back over to the stable branch from exp, I made no changes with the 1.07 mission folder, just to see what would happen. It worked reasonably well, so I tried to replicate the bug on my server. No bug to be found, no matter how hard the rain or how full the barrel.
Then I replaced all of the updated files in my mission folder with the new 1.08 versions. All the files with a modification date of June 30th. I load up the server, make it rain with the COT mod, and there's the barrel sound bug.
I considered if the CF and COT mods were involved, but given that I frequently ran into this on the official exp branch, I would say mods have nothing to do with it.