There are some (all?) large turbofans which emit a distinct "growl" (some people call it a "buzzsaw" sound) at takeoff thrust. What causes it? I've seen it explained (as in this question) as the fan blade tips going trans sonic, but it doesn't account for the pitch, or so I believe. A main fan has something like 20 or 30 blades, and must be spinning at several thousand RPM at takeoff power (maybe on the order of 4,000); a blade tip would then be passing a given fixed point at around 80,000 (or more) per minute or 1,300 (or more) per second.
The musical middle C is only around 260 Hz, so any blade noise should sound like a high-pitched whine, not a low-pitched growl. You can often hear "beat" sounds due to two or more engines running at almost the same speed, but the buzzsaw growl is distinctly different. So what causes that sound at that pitch?