Torque Converter Bolts Backed Off?

[clockdoc]

I've put a little over 600 miles on a newly built 355. I started with an ATI short block and used the top end components from a 327 that ate its thrust bearing and roached the main webs. Early on there was a "chirp" that showed up intermittently, and always after the engine was warmed up thoroughly. I was never able to determine where the noise was coming from. I replaced all belts and corrected one belt alignment issue and there was no change. Following a very long traffic jam last weekend, on an 85 degree day, a much nastier noise appeared. It is a pronounced, knock/rattle that seems to be coming from the rear of the engine. The noise starts as soon as the engine is started and calms down a little after idling for a few minutes. When put in gear, it gets much worse. A light load on the engine quietens it down and, as long as there is a load on the engine, it is quiet. It runs great and sounds good on the road, but gets noisy again at a stoplight unless the idle is held up slightly with the accelerator.

I have concluded that the converter must have loosened up, but have not had time this week to get into it. I plan to get back to it this weekend. Does it sound like I am on the right track? Any words of wisdom?