There is a load noise nearing the end of flushing. I really don't know how to describe it. The toilets flush fine but right at the end of the flush, just before the tank starts to refill there is like a big thud. Kind of as if there is a big air pocket, or as if the there is a clog that just got released. I don't think there is a clog, since the flushing process seems to work just fine. Any idea what the problem may be?