Strange sound while in gear

Hey guys I just bought a 2000 4runner sr5 4wd and it's awesome! It's been well maintained and drives great! I love it already however when I put the truck in either reverse or drive I hear a strange sound coming from the motor. It doesn't make it in neutral or park. It sounds like it would be belts or a pulley tentioner (almost sounds like someone threw rocks in the belt system)? It's only then but performs flawlessly under every other circumstances. The timing belt, water pump, and all seals were recently done by toyota dealership. Truck has 191k miles. I would put up a video clip but the noise isn't loud enough to pick up through the camera. Could it be the belts or should I be worried about something else? Any help is always appreciated!

 

Anyone?