Odd Noise

I have a 73 Duster 340 all numbers car. I had the motor professionally built. The transmission gone through. I have everything installed and hooked up. When I started the car I have a rattle. At first I thought it might be a sticking lifter, rocker, or valve but checked it out and all seem ok. I got to listening to the rattle again and it kind of sounds like it might be coming from the convertor or flexplate as in the bolts hitting something. I used new OEM bolts for both. I don' want to run the motor until I can figure out what the noise is. Any suggestions?