If "the operating noise" is the cooling fan for the magnetron, it keeps going when the time is up to cool it down. Mine does the same, but actually says "COOL" on the screen when it does so.
It's more reliable if you connect the cooling fan to a thermostat rather than the power switch. Otherwise the hottest time is actually after you turn it off (as heat from the inside leaks to the outside but is no longer being cooled by the fan.)
No doubt it was some combination of lower power, physically larger or simply more prone to failure. Do you suppose the designers go to the trouble of a cool-down timer just for the hell of it?
I always use Reverse Hanlon's Razor, so I suppose they do it in bad faith. /s
Actually I guess they both shave off weight/cost and design for built-in model first (that are naturally more heat-constrained) and make standalone variant as an afterthought.
