There is an EMS 135 that flies over the House quite often....and I am beswitched to tell you I am hearing "Engine" noise. I can identify the beat of the Rotor depending upon the distance and direction it passes by....and I would probably imagine I am hearing some of the Fenstron noise....but as to exactly what sound other than the Rotor Blades I am hearing....I would not offer a guess should an Investigator ask me.....that with having been a Helicopter Pilot since 1967.
I would be able probably to state if it sounded as it normally does....but even then that would be a subjective opinion not based upon any empirical observation.
For a complete Novice to look up into the night sky....with all sorts of City Noise going on around him....I doubt very much they could do any better.