I'm wondering what the source of a very high pitched, sometimes annoying, whistle is on my 2003 Dodge Dakota. It's done it since the day i had it, (2nd hand, but was in beyond excellent condition), and only seems to do it during the cold winter months. With that being said, my only guess would be something to do with when i run the heat. However, if i let off the gas, the whistling sound rapidly fades away and will only about half the time start again when i step back onto the accelerator. Any ideas?