Rear end noise

[Champ]

The car is an MX5 NC Roadster 1.8 Venture edition, 2012.
The problem is noise that seems to be coming from the rear end, which I would love to diagnose and fix. I appreciate that it is not easy to identify a noise under the car that can only be heard while you’re driving it however, here goes.

Noise can be heard only above about 20MPH. It’s high pitched - a cross between a whirring and a rasp. It sounds metallic. It’s not a rumble or a whine (wheel bearing or diff). It increases in volume as road speed, not engine speed increases. There’s no difference while accelerating, coasting or on overrun. It’s intrusive at speeds above 50mph.

Can’t hear the sound when the car is run with rear wheels off the ground.

First thoughts were that it was tyre-related. Swapped tyres with another MX. No difference to the sound.

2nd impression was that it was coming from rear/left side. Wheel bearing replaced. No difference to the sound

Next consideration given to diff. Oil drained. What came out looked good. Oil replaced. No difference to the sound.

Transmission professional brought in who thought that it was a diff problem. Diff removed and opened. Concluded that pinion bearing/s worn. Replaced these. No difference.

Don’t know where to go from here. Would love to hear ideas/suggestions. Many thanks.

[Pops]

Might be no harm to check the heat shields, sometimes they can vibrate and make noise.

[Champ]

Turns out that the problem was caused by noise coming into the cabin through soft-top drains.
Problem solved by fixing flaps by rodding with a net curtain wire.