So the car fires right up and knocks like hell while idling (+/-700 rpm) when given any amount of throttle even a 100 rpm bump with the pedal it goes away and runs smooth as a kitten. While driving it runs beautifully until it returns to idle (I e. Redlight) and the loud and I mean loud knocking returns, and vanishes again when I go.

Any ideas?

 

Check the torque converter to flexplate bolts , I've seen an instance where those being loose caused a rapping sound only at idle like you describe ....

 

Could be any of a number of things. You'll just have to track it down by listening all over/under the engine at idle. How's your oil pressure; any difference between now and before this noise began?

 

Also the flexplates like to crack all the way around the crankshaft bolts.