[Geezer408]

There is a GM bulletin on this. It is caused by a brief change in engine settings in cooler weather on warm up when the engine gets to one tick below horizonal on the temp gage. The effect is about 5 exhaust pulses that accelerate the warm up of the cat convertor and reduce emissions. I read the bulletin and was given a copy. You will more likely notice it at a stop light at idle as the exhaust system then is at low flow low pressure. It has never happened under accleration or at highway speeds for me. I suspect it may cause a tap in a slightly loose exhaust baffle but could never get my dealer to check beyond the bulletin. I agree it is a poor "normal" standard for a Cadillac but otherwise I love the car.