If its not coming from your engine, get your tranny checked out. Take it to a tranny repair shop and try to get them to do a free diagnostic on it.
I had the same problem with my tranny and at first i thought it was my clutch also. After replacing the clutch, I experienced the same chattering noise soon after. I have pictures of my open transmission box in my other post. Good luck and I hope it's nothing major with the tranny.