Making the heartbeat reminded me of the repeated discussions of the Moloch whisper. Fiddling with the audio, I believe I have an idea how we can do this. There is a moving sound world even that is tied to a mobile entity. This entity will be the MolochWhisperEntity, which should have no hitbox, be invisible, should locate the nearest player within one chunk of its spawn and then plan a flight path from somewhere behind the player. I suggest 120-150 degrees or 210-240 degrees, above or below by a similar 30 degree range flying to a position on the opposite side (above->below and vice versa). Upon reaching the start of it's path, it should start to emit a MolchoWhisper sound (at a varied pitch, possibly varied volume) and repeat for the duration of the arc.

If the player moves, the MolochWhisper should plot and adjusted path along the predicted vector.

If the MolochWhisper cannot find a player it should expire. If it reaches the end of its arc and it has not finished whispering, it should fly away from the player (causing fading audio). If it does not complete its arc because the player has moved and it's required travel becomes too long, it can expire when it finishes its whisper as well.

MolochWhispers should not spawn naturally and should not drop loot when they expire.