I've had it on a skoda octavia that had a flapping noise with a vibration in the car and it was the 2 rear fixings of the undertray missing.
When the car reached about 60mph it started and stopped once you slowed down.
The diesels have a different undertray to the petrols and fix in to the alloy subframe.
If you look under the car you should be able to see if the plastic tray is secured at the rear.
Ditto. On a C5 sounded like the engine was falling out. Undertray had been damaged by something and was flapping around. Was hard to spot when stationary as it went back into place.