2 months ago out of no where my transmission thud on the downshift. All the maintenance had been preformed on this car from mile 1. Never missed a service of transmission overhaul. I took it to 3 dealers. Aamco said transmission. Auto tech said transmission. Lexus said control box or transmission. Lexus wanted to charge $6,200 to replace box and transmission. I went with /auto tech. I purchased a used transmission and paid $1300. They put it in and I drove it today with the same thud problem. Now I have to take it back to Lexus for and $1,800 control box. This sucks. I heard there was some type of special maintenance on all 2006 sc430 transmissions but I can't find it.