Believe it or not, this might be normal engine noise. especially seeing as how you said it was like that since you bought it. the 2AZ-FE 4-cyl is a somewhat noisey engine, along with the 1GR-FE V6 out of the tacoma's.
my tip for you is when you go to have the dealer look at it, DONT TELL THEM IT WAS LIKE THAT SINCE YOU BOUGHT IT, because they wont even look at it and just assume that its normal engine noise. go in there and tell them that you just noticed it, that way they actually take a look at it.