TC
So he tries to identify the source but the noise(s) and (slight) drop in Nr troubles him to the extent that his best course of action is to remove the stalling engines. So he sets himself up for an auto and chops the throttles (all of this has taken seconds to decide and all the time the engines are popping and surging). He chops the throttles
If the NR was decaying for what ever reason, why would any one want to reduce the throttles?
Surely if you decide to auto rotate (and you still have tail rotor) you would want to have engines assisting? even if they were stalled, they would most likely recover whilst descending?