The Clayton uses the 3" flush valve and trap and dumps a flume of water mostly down the front of the bowl to clean it out quickly - this takes 3-4 seconds and is moderately noisy (although nothing like the cacophony produced by pressure-assisted flushes on some other types), and then quietly refills the tank, which takes only 15 seconds since it doesn't use much water. The Kohlers that use "ingenium" flush swirl the water around as it flushes, taking longer to empty through a 2" valve, but quieter because of it, and also less likely to leave paper stuck to the side of the bowl than with the Toto. It doesn't perform as well and stay unclogged as the Toto when faced with "challenging situations", but eat carefully and there won't be many such situations to concern yourself with. In normal use, the low-end Kohlers (i.e. Wellworth) work great, their parts are also cheap and more readily available than nonstandard Toto stuff.

I only noticed the sound when my Totos were new - mostly because it was a different sound than my previous toilets made.

My mother's Toto developed a high pitched squeal after a few years. She finally determined the cause and fixed it but I don't remember the details.

I've had an Eco Promenade in for a few weeks. The flushing noise is no louder than the 5 gal Kohler it replaced. Because it uses a lot less water, the refilling noise (which is much quieter) stops much sooner. Lots of vague qualifiers here but the bottom line is -- I likey :)

I have a young grandson, 2, that has never been bothered by the sound of our Totos. They are a little louder than the 26 year old Kohlers but take much less time to flush. The (louder) noise only lasts 2-3 seconds.
Monica