This topic has been addressed many times on this and other forums. I have seen (and often not fully understood) some deep technical analyses, incuding manufacturers' research labs, since this is a significant component of take-off noise. (-And all that extra mechanical energy might have adverse effects on blade and liner life.)
Answers generally seem to include the notion of fan tips going supersonic, sometimes with complications vie reflections from stator structures. But the origin of the specific low-audio frequency is not addressed anywhere I've seen. How does a many-bladed fan spinning at a few thousand RPM generate a (harmonic-rich) fundamental around 50 Hz?
In the past few years, I've submitted a possible theory to a couple of other forums, but never had it accepted by moderators. I'm hoping that this group might be helpful.
Thinking about potential acoustic resonances in a turbofan engine, one presumably wants to find the structural element of greatest size in at least one dimension. The core might be considered to be as long as the whole engine, but there are many interruptions in the flow path. The bypass casing, however, might make a nice organ pipe at about the right frequency, excited either asynchronously or as a deep submultiple of the rotating blade-shock frequency.
One phenomenon that this might explain is the slight drop in bass note when power is reduced after takeoff. Fan and core shafts presumably slow down very quickly. But if the main sound is from a pipe resonance, that shouldn't be affected. However, reducing RPM also reduces compression in the bypass channel. This presumably reduces temperature, hence speed of sound, making the pipe acoustically longer and lowering resonant frequency.
Is this silly, or perhaps obvious? The idea may appear elsewhere, but I haven't found it.
Thanks for your thoughts.