I wondered this for ages until someone pointed out to me that it's the intake on these cars. They said it gets louder when you add a pod filter instead.
Either that or you accidently bought a TRD with a smaller supercharger belt. hahaha. But check little things, are there non-toyota wheels on the back, that might be rubbing? (happened to me when I bought my Oreo. Last owner put some massive bogan wheels on, and they were so wide that when accelerating, the weight shift to the back caused them to rub and chafe inside the guard. When I eventually bought genuine Toyota Aurion ones it