I had a big skid today.

My tyres were flat spotted, and then i thought, when this happens to a real car, you can hear a light thud each time the wheel turns around. Surely this can be simulated much like the wind sound as you go faster ?

When you got a really flat flat spot, you will notice a vibration when driving around. At higher speeds it gets less noticeable but it already is there in terms of force feedback.
No real need to implent a thud each time it goes around, seeing your force feedback will do that.

In a race car the wheels are spinning too fast to hear a thud noise. At lower speeds possibly, but if you're hearing that thud than you're probably not going fast enough...

Yeah, you definatly notice a vibration at higher speed in the GTR and OWR cars when you have a flatspot, but not so much in the slower tin-tops.