why are aircraft engines so noisy?
The noise is known as a running engine - not a thing heard often on Jabs...
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In answer to the OP...
Thin walled cylinders (for cooling), minimal muffling (lightness), propeller vortices, torque pulsations transmitted through prop (especially 4 cyl), low rev/high load, engine mounted to firewall (rather than subframe as in a car) allowing drumming of firewall, etc....