I don't drive this car a whole lot as I have other vehicles, but my wife drives it most every day. Noticed the last few times I jumped on the gas I got a rumble that lasted only a few seconds when I was accelerating moderately hard. Seemed to come from the back, but not sure. Could be related to shifting, but again, not sure. May do it mainly when turning. There is a two lane highway near my house where people coming into town from the farmlands to the south frequently are moving at 60-70 mph so naturally I want to get up to speed quickly when I turn onto it. Just curious what this could be and if I should be worried? Car is under the Cadillac top tier extended warranty so I would hope it would be covered. I'd just like to have some idea what is going on before I take it in.