2005 Toyota Sienna XLE Limited AWD mystery noise under front seat

There is some type of rattling/creaking noise coming from under the front seat area in my 2005 Sienna AWD (98K miles on original exhaust system). The noise occurs during stop and go. For example the car is at a stop, I then press the gas and as soon as the car lunges forward, a creaking/ratting noise occurs (0 to 5MPH). I jack-up the car and went underneath to see if it’s a loose heat shield, but all 3 heat shields above the entire exhaust system seems to be in good working order and no rust or looseness. There was a heat shield (under front seats)by where the pipe (Y pipe) splits between the catalytic converter and front exhaust manifold that was loose because the 3 clamps holding it was broken due to rust. I removed that loose heat shield and then drove the car. Again, the noise is still there. All exhaust pipes looks to be in decent condition. Of course with some surface rust. I'm completely baffled as to where this rattle/creaking noise is coming from. Again, the only thing I am sure of is its coming from the back of engine/front seat area. This noise has been around for the last 2-3 years. So it doesn't affect the drivability and the minivan has not had any check engine light.

Anyone else ever had this noise issue? Any idea is greatly appreciated. Thanks