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When starting the car today I noticed as it’s cold it idles around 1k and I could hear a vibration noise that I don’t usually hear. https://filebin.net/dpl2b2t05pkgbqee

Listen to 3 seconds when I increase revs to 1k when you can hear it. It doesn’t happen at 700rpm, only from around 1k and then goes away above 1.1k.

Could it be the alternator, or loose underbody cladding which is vibrating at a certain frequency?

I think I found what’s causing the noise. Is there meant to be a screw or fixture in this black hole?

Update - found the cause.

If anyone experiences a metallic vibration noise under the car, it is the exhaust heat shield above the rear bend of the exhaust pipe where the heatshield spreads out towards the front of the fuel tank. I bended mine slightly and the noise stopped.