I had an allignment done a bit ago and noticed afterward if I take the car out of gear and coast from about 50 down to 40 there is a slight high frequency vibration in the steering wheel. It's hard to describe, but the vibration is really subtle but fast and goes away below 40. If in gear and slowing down it's not there. Ideas?

 

wheel balance and/or tire pressure

 

Badly balanced wheel/wheels, had it in the past on a different car.