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According to another poster on the Club Lexus forum, the tank could be the intake resonator. It's intended to reduce intake air noise.

I verify the 2 black plastic snorkel things connect to the Air Filter housing, not to the mysterious tank. There's no room the get my hand in and see if any hoses connect to the tank. I knock on the tank to see if there's any liquid but it seems empty. Who know if it should contain some liquid ? This bugs me that I can't figure what the tank is for. Does anyone have access to the repair manual for 2009 RX 350 (or 2008 RX 350) ? Maybe you can figure out the purpose of the white tank below the battery. PS: My 2009 RX 350 has Tow Prep package which has Heavy Duty Transmission Cooler, Heavy Duty Alternator and Radiator. I don't know if the mysterious tank is related to the Tow Prep pkg.

The link above was for the 2010 RX 350 which showed the coolant reservoir is on the driver side, near to the battery. Here the link for 2009 RX 350. Page 313 shows the coolant tank is on the passenger side, next to the engine oil dip stick. https://secure.drivers.lexus.com/lexusdrive...l&locale=en

Here is the picture. The red arrow points to the tank in question. Air filter is top left. Radiator is on the right. Here's the close-up picture. Red arrow points to tank.

Windshield washer fluid tank is located on the passenger side. The battery & the unknown tank are on the driver side.

Just bought the 2009 RX 350 Looking under the hood, I saw a white plastic tank under the battery. The tank didn't seem to have any liquid in it, but I could be wrong. I looked to see if any hoses were connected to the tank but the view was blocked by other things. Does anyone know what I am talking about ?