Since last patch I have similar issue. Anytime I'm near helicopter with engine on (or piloting one/flying as passenger) game sound is stuttering, I'm also getting huge fps drops (down to 1-5). Game is now literally unplayable when helicopters are around or if I'm inside of one.
I'm not running any mods.
Before the last patch game was working just fine.
This happens both in MP and single player.