I had this rattling engine noise upon startup for about 3 years already. Read a lot about the same concerns on Toyota Sienna forums saying that it was an oil sludge issue with the engine design or a vvti issue, etc. Brought my van to a Toyota dealer (where I bought it brand new) and was informed I have to pay for them to check on the problem. Decided to get opinion from other car shops and unfortunately getting different information. I tried using some oil additives to no avail. I am almost giving up and just have to deal with the startup noise hoping nothing more serious happens to the engine.