I just installed a Jacuzzi toilet. After flushing it I began to hear rapid dripping under the floor into the drain pipe. I have checked under the floor, no leaks. I put my ear to the pipe. It starts fast and after about five minutes it stops completely. I noticed also after it flushes and refills it drops down just allittle. Flapper is fine. Fill level correct. Fill line into fill tube isn't running. No leaks on floor or under. What else could it be?