Electric Blower
Hi FDR, you mentioned that the 1770 hz is not a multiple of a 400 hz supply. If the blower motor is an induction motor then it will have a slip percentage. if this is 12% then that gives a possibility of 2000 hz, which is the 5th harmonic.
Could you calculate the engine spooling down speed from the descending pitch tone..? It must have something to do with the number of blades on the rotors.