2009 Sienna click sound upond startup

[zcb1999]

I have a 2009 Sienna LE, bought new from a dealer. Everytime I start the engine and let go for 10-20 yards, it gives a click sound once. This happens no matter I padel the gas or not, no matter I apply the brake slightly or not, no matter I go forward or backwards, and no matter how long I wait after starting up the engine. After this click sound happens, if I just park the car without shutting down the engine and later let it go, no sound.
I went to 3 techs in 2 dealerships, the 1st tech told me that the chasis (or something like that) was trying to catchup upon starting and he can do nothing to it. I asked that I don't hear this sound for my other toyota vehicles and he said that every car is different. I didn't feel good at this answer so I went to another dealer, the tech there told me that it's normal and "I won't worry about it"; I went back to the 1st dealership the other day for oil change and asked another tech for this issue, I was told that it's the ABS trying to engage upon startup.
I drive 3 of my company's 2009 RAV4s from time to time for business trips, and I owned an old camry before. I have not heard this kind of sound on any of them.
Any thoughts? Thank you very much.