Not my car for starters here, but one that came to me.
The '00 GTP sounds horrible. The knocking is too bad to be a coupler, its differently something metal. He says if you give it more throttle, the knock goes away, or quiets down. I have not done this yet to see, like on the open highway or what have you.
Thinking, if thats the case, wasn't the general rule on our 3800's if the knock is there and gets louder as you give it more throttle its probably a rod knocking, and if you give it more throttle and it goes away its probably a flex plate that's cracked?
I think that's correct, but we can always pull the cover off and look up inside and see if we can see damage to the flex plate or move it around and see if it moves more than it should...or drain the oil and see what surprises it has in that or even cut open the oil filter and look in that if its internal damage.
~F~