I just had this problem repaired! Rattled and stuff till you revved it and then it went away, drove fine...
Replaced my timing belt and tensioner, no change.
They replaced my flywheel, torque converter, main seal and transmission pump. Problem solved for under $800(yay tax refund!).
My flywheel was literally in pieces and the torque converter was also ruined. The pump had spun a bearing. The mechanic said it was a miracle that my car even moved much less drove 10 miles at 55 on the highway to the shop.
The center of the flywheel had separated from the outer rim. Clanky and rattley...
Hope that helps.