So my car has started making a knocking noise thats a bit to deep to be valve-train, it started about 600 miles ago but hasn't gotten worse. However it does someone resemble the sound of a rod knocking. You only hear it after the car warms up and at a steady rpm after 1500 rpms, you don't hear it under acceleration or deceleration. Any ideas in what it could be? Iv been driving it the way it is with the intentions of getting it checked out but have yet to have the time.