1998 Chrysler Concorde Whistleing under the hood

Just got the car and had it about 2 weeks. The other day I noticed a very audible whistling sound from under the hood. At idle the sound is barely audible if at all. The sound seems to start low then go to a peak then die down to nothing. The sequence may take 5-10 seconds. It never really goes away even though there may be periods where I cant here it for a few seconds. Doesn't matter how fast or how hard I get on the gas but always more noticeable under acceleration.
One more thing, what would make the rear defroster stop working? Any help would be greatly appreciated.
Morrell