I've had my 2011 Altima for about 3 months, and almost the entire time I've had it, it makes a loud tapping/ticking sound from the engine when the engine is cold. The noise is worse when I accelerate. Once the car warms up, I don't hear it as loudly. The dealer said all 2.5's make that noise, but I can imagine why an engine would make that kind of noise if nothing was wrong. Any suggestions?