2005 Sienna grinding sounds?

[Roots]

Hi everyone, I have a 2005 Sienna LE, about one yr old now. When I shifted it out of park to R, I hear a really loud grinding sound, like the sound of rusted metal grinding against each other. The sound from older cars as you shift out of park, if anyone knows what I'm talking about. Anyways, this happens once and won't happen again during the day but a day or more later, it does it again.

Would anyone know what's causing this and should I be concerned about it?

Dealer can't find anything wrong with it.

Thanks