whistling noise

my 2001 dodge ram 1500 5.2l makes a peculiar whistling sound when under load and when the outside temperature is 0 celsius or lower the sound goes away when you let off the gas or when you put your foot in it. it also is
not as prevalent when the cruise is engaged.the cruise before never seemed quite right in that it would really hammer in and out of a lower gear and now when it is engaged the acceleration is alot smoother go figure. so is it a throttle problem or transmission or some type of vacumm leak help please.
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