RCM and Aviation Modeler International have had articles explaining what goes into the sound from the planes (And probably much of the same things go into boats).
Airframe (Hullframe?) resonance, prop speed, intake airflow all contribute. An effective muffler can lower the exhaust sound to the point where these other components are the primary noise source. High speed engines or motors will be loud, no matter what.

The quietest plane, outside of gliders, we ever had at our field was a scale F6F with an unmuffled ST 60 turning a 13-4 prop. The exhaust and intake noise was effectively suppressed by the large cowl with dummy engine forming a sound box and the relatively slow turning large diameter prop. One of the loudest, outside of ducted fans, was an electric weight lifter test flown by a college team. The brushless motor was turning a 16 6-10 prop fast enough to send the tips at least transonic if not supersonic. The tip noise hurt your teeth. The couple turbines we've had seem to measure about the same as some of the sport 45s to 60s, at least while they are in the air at full throttle. They seem surprisingly quiet at idle, but I haven't had the opportunity to measure one yet.

In response to a comment about my measuring the Quadra at 125 dBA, an IMMAC member responded that "Radio Shack told him their meter doesn't hold it's accuracy if it's hot out, so we can't depend on what it says!". I'm tempted to do some temperature testing at work, raising and lowering the meter temperature in one of our environmental chambers and re-testing against our calibration standard. I doubt very much there is any significant difference within our normal summer temperature range. Or I could borrow the lab B&K, which is certified between 20 to 120 F and take that to the field.