This is normal. The sound is made when they are trying to produce saliva. Sometimes when washing, they need extra saliva to really get their fur wet. This often indicates the presense of a lot of dust in the cats fur. Indoor cats will go in all the little spaces that escape the regular household cleaning routine, cats that go out can find dust anywhere. Also, some dirt will stick to their fur, so they really need to dampen it to get it clean. You should check their fur regularly, in case there is something stuck so well that they can't remove it, in which case it's time for scissors. It is also a very effective flea catching technique. A healthy cat will deal with fleas so effectively that most owners never know that they have them. You only become aware of fleas if your cat has become ill, or elderly. Please DO NOT spray your cat with anti-flea products. Most of them contain nerve toxins that many vets believe will shorten the cats life. (Notice that the can warns you not to get it on YOUR skin.) If you want to help your cat deal with fleas, get a flea comb. Dust is the most likely, and the hardest to detect in the cats fur. So don't worry.