There is no question that the sound I heard could certainly be flushing. For example, when the passenger in Bedroom A flushes, you certainly hear it in Bedroom B.
But you have to take into account -- and I am not making this up -- it happened literally every 20 seconds. That's three times a minute, on a regular basis, for the whole 18 hour trip. Now that can't be random people flushing, no matter how often. Too regular and consistent. It's got to be programmed to operate that way.
So if it's the waste disposal system, then it must be pumping stuff from point A to point B at a consistent rate, constantly. Independent of actual flushes.
Yet at the same time, the conductor told me it was the HVAC circulating air to the entire car. And as I mentioned, it did sound like an AC cycling on and off.
I'd like to at least try to explain this thing intelligently when I contact Customer Relations. Right now I'm uncertain what I was hearing. Thanks all for the information offered!! Really helpful sorting through the options.