When I start my car up, occasionally, it will knock when I turn the air on. Its not the motor cause I got out of the car and I didn't hear it but I heard it again when I got back in the car. I hear it from the vent. Any ideas of what it is?

 

Any way you could post a video with sound? That would help a lot. My initial thought is a busted exhaust hanger.

 

You can hear all sorts of things in a closed cabin that you can not hear from outside the cabin. Yep ...... "could" still be a mount.

 

I will try to get a video of it.