What causes the "growl" of some large turbofans at takeoff thrust?
There is a wide range of harmonics generated by the fan, descending all the way into the lower sound spectrum. This article is about sound insulation of a turbofan shroud, and contains sound measurements of an actual turbofan in a ground installation at full TO power:



The article seems to be Google translated into English from Russian, but the observations made are very valid:


  At power regimes with Mach number M>1 at the periphery of blade wheel there is a well- observed shock wave noise appeared on harmonics of rotor frequency that could be seen at fre- quency range between the first rotor frequency and fan BPF. Shock wave noise has a high sound power level due to the used lemniscate inlet duct without lining and waisting.


We presume that "waisting" is meant as in: application of a varying cross-sectional shape (waist). The blue lines in the plot are the harmonics of the rotor frequency. There are 16 of them in the first 1200 Hz, in a linear scale this implies that the distance between blue lines is 75 Hz. 

Significant sound peaks can be found at intervals of 75 Hz, immediately starting at the first harmonic line of 75 Hz. Note that the fan Blade Passing Frequency is at around 2,500 Hz, but that the frequency spectrum below the fan BPF is quite densely populated. The dominant sound that is observed on ground from an aircraft turbofan is of course dependent upon the position, distance, inlet shape, acoustic insulation measures etc etc. It makes a lot of sense if most sound insulation efforts are invested in the loudest spectrum, and that the low buzz sounds are regarded as a low priority for suppression since they are not very disturbing.