Godzilla-T78

Ok, got my car together, got it running, everything is going great.. Go to add some coolant before I take it on its maiden voyage and this loud crazy sound starts when I start the car up.. I immadiately turn it off. Confused, i turn the motor over by hand, nothing, I pull egi fuse and pull plugs and run it for 30seconds solid on the starter.. nothing.. I put it all back together, start it back up there it is again. At first I was like oh **** there goes an apex or somthing of the sort but my compression is perfect. I dont know what else it can be. Only happens when the car is fired up.. I thought maybe somthing in bellhouse maybe. My rear iron where one of the bolts to tranny goes was broke off so figured maybe a nut or somthing fell in there, or perhaps a clutch nut etc.. I havnt jacked it up yet to check.. Any ideas?

Btw, car runs fine even with the noise.. perfect idle.

Godzilla-T78

follow up: throw out bearing wasnt in all the way.