At 50 mph or more I have a noise which sounds like something is flapping in the wind and banging against the underside of the car. I wondered if it was mudflaps (it isn’t) or even a seatbelt or similar hanging in the wind. It isn’t. There’s nothing obvious hanging down, or loose.

It’s very predictable and definitely road speed related, not engine speed related.

Has anyone else had this problem? And if so, what was it?

Thanks

2014 Freelander 2 diesel 160 HP manual gearbox

Check all the heat shields, as they are known to break around the mounting nuts. Large "mudguard" or "fender" washers can be sandwiched between the nuts and heat shields to increase the diameter of the mounting nuts, securing the flapping heat shield again. Hyundai Ioniq 5 Ultimate. The family car.
2009 Rimini Red SE TD4. Gone.
2006 Tonga Green i6 HSE. Gone.
Audi A5 convertible, my daily driver.
1972 Hillman Avenger GT, the project.

Thank you! You were spot on. The one next to the prop shaft had torn. I’ve jury rigged it for now. Noise gone away. Much appreciated.