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99 Jetta Rattling

My 99 Jetta is rattling. The noise comes from the front center of the underside of the car. It started out only when I was accelerating and would disappear when in neutral or after reaching 40mph. Now its when its idling and driving at any speeds under 40mph (I think it just gets drowned out above that speed). I looked under the car and saw that there might be a piece causing the noise but I can’t figure out what it is. It seems to be in-line with the exhaust system and its a metal, cylinder looking piece that has uniform holes in it. I can try and get a picture if that helps. I’m just trying to figure out what is causing the noise, if I can fix it easily or if this is going to be a pretty expensive fix. (It has 165k miles on it, is a 2.1 gas engine with the new body type if that helps)
Thanks!

It may very well be a heat shield to the catalytic converter or muffler. Sometimes mounting ears rot right off or hardware falls out. Pop your head under the car and give all things that look like they are part of the muffler a good shake. I bet one of them is loose and resonates at certain engine speeds.
If that ends up being the cause and you can’t remount it properly, you could use a very large band clamp (home depot plumbing department) and wrap it around the shield to hold it together to make it stop rattling in lieu of spending actual money and replacing the shield.