My CTS has 62k miles. recently when I start it in the morning and reversed out of my driveway there is a noise like teeth not meshing correctly for a few moments. Today when I heard it I came to a stop and it continued for a moment. It even brought down my rpms for a moment. Once I get on the road it lessens and after 5 minutes I can barely tell. I put it in nuetral and the car revs smoothly and like I said the noise has happened shortly while the car is stationary. That is why I am thinking the transmission. My first thought is to flush the fluid since that would be easy. Any ideas?