My experience lies in the 'outside the helicopter' arena.
I was learning to skydive in Cyprus in the mid 1970s, we dropped from a Beaver near Kingsfield airstrip, close to Dhekalia, where the Army Air Corps operated Bell 47s.
I was on my second descent of the day, at about 2000' when I heard a chopper and deduced it was below me. In those days we had round chutes which were not very manoeuverable and try as I might I couldn't turn the thing to look for the chopper, the noise of which was growing louder and louder. I was sure I was about to drop throught the rotors of this thing when the noise started to recede, I never saw it but I sometimes still hear it...