Don't know if I can help specifically, but a few pointers on engine noise at low powers:
-535s especially, have a noticeable hoot at ground idle powers, put down to combustion instabilities. Combustion rumble is most audible at low powers.
IP and HP turbine (rotor/stator) blade interaction tones, which are normally at too high a frequency to be heard, will drop through the audible range when spooling down. The relative Mach no.s will still be high enough for this to be significant.
Handling bleeds may be open, jettisoning HP air into the by-pass duct, though this will be broadband noise usually.
Hope this helps.