That's the problem. Usually when a water pump goes out, it starts leaking. But I've been told that it's not uncommon that the vanes erode eventually which just cuts down on it's ability to move the water. In that failure mode, it doesn't leak ... or make noise. Worst of all, the only way to tell if that's the problem is to remove it & inspect it. If you're going to go through all that trouble then you should replace it.

BTW, even though I've been told that it's not uncommon, I've NEVER known anyone that has experienced that problem.