I've noticed recently my American Standard has had a noise coming from it and it originates from the drain tube. When I pull up the safety switch it stops. It's some sort of vacuum coming from the opening there and the noise seems to be just a bit of a thumping sound. I guess the vacuum is pulling on a panel in the air distributor. I only seem to notice it during the end of the cycle. Should there be any pressurization of the drain tube that would cause a vacuum pull at the safety switch T? It doesn't seem to affect the function of the unit but is kind of annoying. Would any type of blockage going back to the unit from the T be a cause? The unit is about a year and a half old so I wouldn't think it's clogged. I haven't called the installers yet, didn't want to bring them out and it ends up being normal operation and I'm just now noticing. Any advice is appreciated. Thanks