I don't know if it's a problem or not, but I was driving my car this weekend, and had an open space of road and wound it out a bit in 2nd, to around 5k rpm then shifted to 3rd. While I was accelerating I heard what sounded like the valves rattling. Has anyone else heard this before? Is it something I should worry about, or is it normal when the engine is being pushed. I was just curious because I hadn't heard this noise before.