Every 20 seconds or so, we get a sound from the bath drain that sounds like water draining, or gurgling. Even if the tub has not been used for days. The tub faucet does not leak. I have opinions this may be a problen with the vent pipe. It can go on for hours, then disappear for awhile. But it always starts again. House is 5 years old. This is in the upstairs bathroom. Any ideas what might be causing this noise, and how to correct it?