2006 Hyundai Elantra Loud rattle noise

I had a similar sounding problem, but not evident in the warm months and only upon accelerating from a cold engine in cool/cold weather conditions. Are you sure it isn't the engine and just coming through the windshield like a rattle in the dash? I thought it was a rattle in the dash too, but it turned out to be coming from under the engine cover.

If it is the dash, it is probably something loose in the ventilation system, or even a buzzing sound from a heat shield or what not (that is what they first suspected on my Elantra back in 2001). If it is the engine, perhaps the valves or the tension slightly out of whack on the belts.

In my case, it could have been as simple as a bolt being lose around the upper or lower engine cover, and not the fact that the belt tension itself was faulty (they never said if it was). In the years before I replaced out the belts, Hyundai did all sorts of things to fix my car's problem, ranging from replacing a motor mount to the thrust bearing washer. They even told me to try changing gas (in case of pinging under load), which in my case didn't work.

I presume your vehicle is under warranty. If so, I would pursue it with the dealer. Don't let them play dumb. I had my car making this noise six months out of every year for over six years and it drove me nuts. As a result, I only drove the car when I absolutely had to and have next to no miles on it (relative to its age). Personally, I won't be buying a Hyundai again because either their service people are the most incompetent or they were trying to run me out of warranty coverage (since I ended up paying out of pocket when I had been asking them to check the tensioner all along and they refused).

Can't say that I have a solution for you. Best of luck that you have a faster, simpler explanation!