Why do I hear whirring noise when I'm about to shift to 2nd gear and the clutch is pressed in?

I have a 2008 Civic with a manual (the 5-speed...can't afford the SI). Sometimes I hear this whirring noise after I've been in 1st gear and the clutch has been pressed all the way in. The noise stops the moment I shift into neutral or the 2nd gear. It's not that loud and I'm not getting any problems with the otherwise, but it is especially noticeable when the weather and/or car is cold. I don't think it's the throwout bearing because I don't hear the noise when the clutch is depressed in any other situation (neutral, 2nd to 3rd gear, 3rd to 4th, etc.). Basically, it only happens when in 1st gear and the is still spinning freely due to the momentum carried over from previously. Should I be worried or is this normal? Thanks in advance!