When I first start up my 2006 Cadillac CTS, the vehicle runs rough and I hear a rattling sound. After a while I drive the car and it seems find. There must be a problem, but I need some type of an answer before I bring it in to Cadillac for repairs.
It could be a vacuum leak as the computer takes over when it's almost warmed up. Check for a vacuum hose off or a split one or broken one. Alot of them are plastick andcan be repaired at the break with a pieceo fhose going ove r it. The rattle may be the heat shield has broken loose over the converter. You'll have to crawl uner it and look.