2000 Chevy Venture Intermittent whining (laboring) sound wh

Engine problem
2000 Chevy Venture 6 cyl Automatic 65000 miles

For the last two months, there has been an intermittent whining sound coming from the engine while the car is in motion. Actuially sounds like it is laboring like a truck up a hill (that sound you might hear just before you change to a higher gear). Well in this case the sound is there, but the gears change as usual, and the vehicle performs as usual. It is only a difference in sound. As I said, this happens intermittently, so when I have taken it in to the shop, they can't duplicate conditions. One day, I had the scenario, so I stopped by the shop. After listening to it in neutral and driving it a bit, they turnd it off and said that it was a hard one to diagnose, but. It could be my water pump needs to be changed, and something is sticking at times, and tht is why the sound is intermittent. They also pronounced my transmission to be okay. I am not sure I follow the logic, so can you explain it, if that is indeed what is causing this sound. Any advice is greatly appreciated. Thanks a bunch.