engine noise 01 Prius

I bought an 01 Prius about 2 months ago, and it was super quiet. Just the other day when I started it was louder than I had ever heard it. It does it when I start it, when I accelerate, and gets louder when I turn the air conditioner on. The check engine light came on not to long after this, and I had it checked, and it said system to lean. All fluids are full, no leaks, and the trans fluid and the oil were changed not to long ago. Any ideas?