Whistle after turn 2004 Subaru off

Hi,

I can't find any info on this in my manual...

My car seems to be operating just fine - nothing unusual at all. However, when I turn my car off, there is a high pitched whistle that continues for a good 5 or 10 minutes. It seems to be coming from the right side of the car, under the hood, but I can't find the exact source.

It just started doing this in the last week or two, but happens consistently when I turn it off the past week.

It has been very cold here, but doesn't seem to be related to the outside temperature.

Any ideas on cause and if this is a problem?

Thanks.