new noise

[yaris9876]

07 Yaris hatch..
This has always run perfect and quiet... almost silent at idle..
We just had 5 days of 15-25 below 0 at night... I started it a couple times only for a half hour or so the whole time... I went on a road trip today and have a new noise... When its idle and you rev it a little
it kind of winds down like on a slight delay after you let go of the pedal. Like a kind of whine following the engine sound...it makes the sound reving up and down..
it sounds like some cheap cars sound all the time....its not a scream
or grind..just a following delay sound with a plastic overtone...
I checked and nothing is rubbing..
I dont like this new development...I am used to perfection from the
Yaris....
What do you think...???mechanics...??