Hi there. My 2008 Frontier 4wd makes a horrible rattling/vibration sound at 3000 + RPM (Just about everytime I jump on it to get on the freeway or pass somebody). I had my mechanic take a quick look while it was up on the rack getting tires rotated/oil change and he didn't notice anything out of the ordinary. Still a warranty item. I just wanted to check here first so I could present the dealer with some possible sources of the sound. Perhaps it's the skid plates vibrating at a certain harmonic range? Just hoping it's nothing serious. Please help.