The only two I'd leave in a car if I had to are the Enya Nova U and the Outdoor Ukulele.
I would also suggest changing strings, if you haven't already. In the process, shake around the uke to make sure something isn't inside loose. Clean the nut slots, bridge slots, etc. Make sure the screws and hardware are all tight enough on the tuning machines. Other than that, I'm not sure what it could be. The braces on the top are molded into the body, so that eliminates a loose brace.
Did you try contacting the people at OU?The Outdoor Ukulele is a very decent, but we have a small issue with ours. An annoying buzz has developed and we cant seem
to fix it. It was our car ukulele for over a year and we kept in a padded Gator bag, I don't think it was abused, but it might have gotten a knock.
I'd still recommend OU, no one else seems to have any problems with theirs.
My buddy uses his everywhere and often plays at the beach.
I would also suggest changing strings, if you haven't already. In the process, shake around the uke to make sure something isn't inside loose. Clean the nut slots, bridge slots, etc. Make sure the screws and hardware are all tight enough on the tuning machines. Other than that, I'm not sure what it could be. The braces on the top are molded into the body, so that eliminates a loose brace.