L.S. has been doing the same thing for about two months now. I never thought it was serious, but since he's not stopping, I have a vet appointment for the 14th. They didn't think it was serious enough to warrent bringing him in sooner. In the beginning he used to stop vocalizing as soon as we turned the cellar light for him, but now he's doing every time he's using the box, regardless of the situation.
Personally, because of how musky he smells, I'm thinking it's a problem with his anal glands. He's not scooting around, but man, does he reek.
Sometimes I also wonder if it's just him waiting too long to get to the bathroom. His stool is normal, and he's not straining, and he's going two times a day, which is normal for him. He has to be around everyone else all the time, though. He can't stand the thought of missing something. I really do think he suddenly realizes he can't wait anymore, and that digging a hole takes too long, so he starts vocalizing. (Never a painful meow, either. He trills like he does when he wants our attention.) Kind of like a kid who waited to long to get to a potty.
I'm glad you're getting your boy to the vet, too. Personally, I always like to rule out health before behavioural. If it's something serious than it's caught early. Plus, it's so much easier to treat a health problem than a behaviour problem. I prefer quick fixes.