Engine racing sound upon engine startup is caused by the high volume of air racing through the PCV valve in the camcover. Normal upon cold or warm startup; with the time of the high airflow dependent upon engine startup temp. As to why it was happening while coming down from around 10K to 5K has not been answered, and there have been a few reports of this happening.
Some believe it could be due to the altitude and temperature change. Didn't cause any problems, and never set any codes.
