Shawk - HPs part of engine
Shawk,
You say that "cavitation noise tends to be high frequency and is likely to be attenuated by the aircraft structure".
The cavitating high pressure fuel pumps (HPFP), that I mentioned might be the source of the reported strange noise, are integrated with, and driven by, the engines.
Lots of local pipework and other metal objects (nacelle etc) in the vicinity available to resonate with the HPFP, plus, a large hole at one end to let sound out.
Would much attenuatation occur and how would the other resonating objects alter the frequency range?
Of course, if your interesting fluidic resonance idea is correct then the sound is important evidence of causation. If it is the result of the cavitation then it is not important.
Regards, Tanimbar