the engine intermittently makes a rattling noise when it is hot at idle. on 2004 Cadillac CTS

the engine intermittently makes a rattling noise when it is hot at idle, usually after getting off the highway. It does not do this all the time but it IS getting worse. Anyone know what it could be and how much it might cost to repair?

2 answers
Had this problem..mine was timing guids
could be a broken heat shield or a bad converter