In most modern turbofan engines, at high engine power operating conditions, the fan tip rotation speed will become supersonic. The noise spectrum from a supersonic fan is very different compared to a fan that is rotating subsonically.
Supersonic fans produce a multitude of high-amplitude tones at harmonics of the engine’s shaft rotation frequency. These tones are known commonly as the “buzz-saw” noise.
One generally needs to sit in front of the engines to hear the buzzsaw noise. This is why, on aircraft with fuselage-mounted engines such as the Boeing 717, most passengers can hear the buzzsaw noise generated. However, on aircraft with wing-mounted engines, this “privilege” is usually reserved for those sitting in front of the wings. Similarly, the buzzsaw can be heard by someone standing on the ground as the plane approaches. However, the effect dissipates somewhat after they are left in the plane’s wake.
In Infinite Flight, as noted by Tyler Shelton, a staff member for Infinite Flight, “The buzz is barely detectable from the flight deck in real life as well. With good headphones or at loud volumes you can still hear it faintly from the cockpit on Infinite Flight (as it should be)“.
You may be able to hear the buzz-saw effect in the most recent teaser of the RR RB211 on the newly reworked B757 that is set to be released in the next update, v20.3.