Sure sounds like an air leak somewhere and that somewhere would be on the suction side, not the pressure side so concentrate on the suction side. It also sounds like a very tiny leak and that's all it takes. You can try smearing vaseline around the pipe joints as it is thick and if you have a hairline crack that should seal it. Then fix it when you find it. Don't leave the vaseline on O-rings or gaskets for any period of time. Vaseline is cheap and cleans off easily. Also, is yours an above or in-ground pool? Reason I ask is for access to the skimmer basket connection. Under a deck can be a dog.
Al