I haven’t had the pleasure of hearing the Dart and a few others mentioned, but the Merlin is a joy. In my area, there used to be a Connie that flew regularly, and was unmistakable. There is a pair of Avanti based locally that sound like nothing else, but I suspect the props are what are making the unique racket. The T-6 was mentioned, but again, the supersonic prop tips. They truly grate on my ears, and I duck for cover when they are flying. The Old Shakey my dad flew, 4 x 4360s. Agreed on the MU-2, but I’ve never heard overflight, only on the ground. How about the shrill T-37? No props disguising that cacophony. The heli world has some very characteristic sounds, but those are rotor noises: the iconic Huey, the no-doubt Chinook, and the Bell 47, often used as the soundtrack for ANY helicopter on the wretched television.