ryucasta,
I was concerned about the aluminum linkeage when ordering my 650 since I previously broke off the lever on a Lee LoadMaster. The Dillon rep said not to worry unless I was a gorilla. He was evidently thinking of you!!
I began hearing and feeling poping/cracking in my press a few thousand rounds back and haven't figured out what it is. Seems to be coming from that same area where yours cracked. When I seat the primer, I get two pops. During the down stroke, I hear one pop when the cases start into the dies and two more at the completion of the stroke. Everything that can be tightened has been. I have decided to just keep on and if it breaks, they can replace it.