On acceleration, and sometimes even at higher speeds on the highway. Took it in twice, and the service manager said they could not find anything definite, but did tell me this model was notorious for leaking coolant into transmission, and he suggested that I sell the car rather than try to have repaired, as soon as possible. Am still driving it, hoping to hold out until all of the other problems it has can be evaluated so I can not lose money just "taking it over the scales"....the "service engine light" has been a major issue as well. Evidently the gas level sensor has gone bad, and it is buried in the fuel tank which requires a complete new fuel pump. Prior to the transmission issues. One brake shoe disintegrated and destroyed lines, etc. Required $850 new brake setup. Worst car I have ever owned. No more Nissans for me.