Strange that it happens at a specific RPM rather than speed. I was getting a rattle from the rear of mine that puzzled me and it turned out that the false floor of the boot was not seated properly because I had too much stuffed underneath it.
With strange rattles, I always suggest that people remove all of their junk from the car, boot, doors and glove box and see if the rattles disappear. 90% of the time they do 😃
B Max Titanium X 1.0 125 bhp in Magnetic with built in SatNav and rear sensors.