Our bathroom vanity sink drains very well, however, after the water goes down the drain, we hear a noise (e.g., "glug" / "glug" / "glug" / "glug', etc.) for some time. I have snaked the drain all the way to where it joins the shower drain and goes down to the basement; all seems clear. We ran two shots of the new foaming type drain cleaner down from the sink, but the noise persists. Any ideas???