Took the car in for a brake and rotor inspection and found nothing. Rear brakes getting thin but nothing to cause the knocking sound. Besides, noise is coming from the front of the car. However, the mechanic doing his full inspection noticed the transmission fluid was dirty and needed replaced. Had that done and the sound is gone, at least for now. Any of you gear heads have an explanation for this? Only thing i could think of is that I am getting that sound when the car is down shifting when apply brakes.