My Mazda was probably typical old-school I4, in that it ideally idled at ~850 RPM, but its idle speed was rather inconsistent (because no feed-back idle-speed control in '81), and slight variations in speed could make a disproportionate difference in noise level, depending on various resonant frequencies. At its best, reasonably smooth, considering its size, weight, etc. Turning on headlights or stepping on the brake slowed the idle significantly, as did adverse temperatures. Pretty much all the same comments applied to my earlier Subaru with its 1267 cm³ horizontally opposed four.
Prius normally never idles much, except when cold. When it stops, it's quieter than any idling V8 or V-12. When it's cold, it idles at around 1200 RPM (not particularly constant, oddly), and makes considerably more noise than my niece's '13 Corolla (which has the non-hybrid version of the same engine) does when idling. I suspect that's because the Prius engine mounts and exhaust are tuned less for quiet idle than for unobtrusive cruising at its typical lowish engine speeds under moderately high load.
Coasting at speeds above ~45 mph, the Prius engine is rotated (as opposed to runs!) at 992 RPM, which is undetectable over road and wind noise.