3.8, automatic, runs great, sounds terrible. Theres some sort of vibration or loud rattling under the hood and I have no idea what it is. 15-60 mph is when the annoying rattling begins. I thought my bird wasn't getting enough air into the engine, so I replaced the intake and come to find out, thats not it. I don't know what to do, as I can't pin point the problem cause it doesn't rattle when idle. Please help me, could it be my front pop-up headlights?
You'll probably have to get a mechanic involved, but one thing you can look at is the harmonic balancer. That's the chunk of steel at the front of the engine that the main pulley is bolted to or cast as part of. That pulley runs the belt that runs the generator and power steering pump.
Most harmonic balancers, (aka vibration damper), have an outer ring and an inner hub, and a rubber layer bonding them together. That rubber often breaks free. Auto parts stores have entire books with nothing but replacement balancers for all kinds of car brands and engine sizes. The center bolt can simply come loose allowing the damper to wobble too. Either way, one potential clue is to sight down the edges of all the pulleys to see if the belt is peeking out at the bottom. All of the pulleys must remain in perfect alignment to prevent a belt squeal.
You'll have to tug on the outer ring too to feel if it is broken free. To do that usually requires loosing the serpentine belt to remove the tension on the pulley.