The early P-51s (P-51, P-51A and the A-36 Apache) were Allison powered and a bit asthmatic - Rolls Royce TP Ronnie Harker suggested bolting a Merlin on the front which led to the P-51B and a quantum leap in performance; all subsequent Mustangs were Merlin-powered.
Duxford hosted an American collector's A-36 at the Flying Legends airshow a few years ago, made a nice contrast with the Merlin variants present.
Edit:
I always thought that it's the radiator housing that whistles, but I have often read that it's the gun ports.