Intermittent grinding noise possible transmission problem?

I have a 2005 Dodge Caravan SXT with 62,000 miles. 3 weeks ago as I was pulling into a parking lot I heard a very loud grinding noise coming from the front of the car, drivers side. I stopped the car and the noise stopped as well. Tried to reverse and got a "shudder". Put the car back into Drive and the loud grinding sound started again. Pushed on the gas and once it got up to about 15mph the noise stopped. This has happed about 5-6 times in the last 3 weeks. Not everytime we drive it, whether the engine is warmed up or cold. Inclines do not seem to make a difference. It does it in Neutral too. We have taken it to a general mechanic who drove it several times, couldn't find anything wrong and didn't hear the noise. Transmission fluid is red, no smells. No check engine light and no codes. 2 transmission places kept the van overnight, drove it several times. But because it is intermittent, they were unable to hear the sound. Yet they told us we probably needed a new transmission anyway. Does this sound right? Has anyone heard of this happening before? Even in Neutral?