Rausky

When I do a cold start and put my car into reverse or drive, it usually lags for a second or two then starts to roll (Applying no gas). Now, all of a sudden, it's starting to grind. Once the car is warm it's perfectly fine and this doesn't happen but has anyone else experienced this? I took it in to MB when it was just slightly laggy and they said nothing's wrong but now that it's starting to grind makes me believe there is.