transmission noise

hi, i have a 1966 mustang 289 automatic c4 transmission. I have a knocking noise that i think it is coming from the bell housing in the trans. I checked and tighten all the bolts on the flywheel, and look that the flywheel and didnt see anything wrong. i also took the starter out and it looks good. The noise started a week ago and when it first started the noise was at low rpm and wasnt to loud. Once the rpm was up i didnt hear the noise. Now the noise it louder and still at idle speed. Once the rpms are up i do not hear the noise. I dont think the motor is drowning the noise out., thank you if anyone can help me out.