My pump on my 53 F100 that I'm restorodding does pretty much the same thing and it whines like a supercharger. I agree with Tony that these pumps are notorious for being bad. Google this "saginaw pump on ford." This is what I'm going with. The pump is about $40 through AZ if I remember correctly. I don't know if Mustang people are doing this swap, but I don't see why it wouldn't work.
[This message has been edited by carnut122 (edited 08-05-2012).]