Much of the noise you hear from any propeller-driven aircraft comes from the tips of the propeller breaking the sound barrier. That won't change between an electric vs petrol engine. The longer the propeller, the faster the tips move.

You're right that a fair amount of noise comes from the prop, and in particular that petrol vs electric won't change this.

However, I do not agree that it's because of breaking the sound barrier. Good prop design will keep the max tip speed below 0.6-0.7 Mach at max RPM and flight speed. It would be extremely hard to spin a well-specced prop fast enough that you'd see tips breaking the sound barrier. In an airplane which doesn't have a prop specifically designed to work transsonic (which AFAIK is pretty much any airplane outside of a handful of research ones), the prop loses so much efficiency as the tips approach Mach that you either actually slow down or dramatically increase the required engine power.