1999 Cadillac Deville Help!

Other Category problem
1999 Cadillac Deville V8 Front Wheel Drive Automatic 168xxx miles

Here's the story. Last Friday evening I parked the Cadillac in my driveway around 10PM. No problems. Overnight it got down to about -25 below. Saturday morning the vehicle would not start. Would crank and sound like it was going to start, but wouldn't go ahead and fire. Monday morning rolls around and I have her towed to my mechanic where he finds the engine was flooded, he resolves the issue, changes the oil and all is good.

But then, he asks me if I knew about the "whistling" sound coming from under the hood. I say no it wasn't there previously and here I am.

When I drove it home the sound was continuous both during idle and on the drive home. Occasionally when driving the sound will go away for 3-4 seconds before it comes back. When I started it today I let her warm up for about 5 minutes, during this time from start up to the few minutes idling the noise was totally gone! Well. Then it came back. Also, when I shut the car down, the sound remains for a few seconds but then "dies out" Electrical? Mechanical? Both?

It sounds like a whistle or maybe like the sound of a teakettle telling it's done.

Please help! Thanks.