Honda- Engine Rev

I have a 2008 Honda Civic. Recently, when I put it in park after driving, the engine with REV. Any ideas why and how to fix?

Rev how? Spike all the way to redline and stay there? bounce up and down? What?

No, not all the way to the redline, probably just to 2000rpm. Doesn’t bounce, stays there.

It might just be the power demanded by the alternator when the cooling fan comes on.