I'm not too sure what it is but it's like something knocking around the wheel arch, kinda like a bit of metal on the rim hitting the suspension or a caliper as it turns (if you know what that sounds like).
Like a train going over rails. gets faster when you go faster.
I only noticed it when playing lfs with headphones on.

I'm not too good with explaining things but i'm sure you can notice it.

could you record the sound maby?

It's a problem in the way the gearwhine sample loops. It's not long enough to allow for a decent amount of variation and inevitably, being a pattern detecting being, you'll pick up on the loop - if a purer waveform is used then it just sounds terribly bland.

You can try this by pressing shift-a and setting the gearwhine to "solo" (thus muting all other sounds).

Quote from scoobyrbac :

could you record the sound maby?

Turn the volume up and go over 90mph.

Use Default as engine sound.

I may recognised this too.
But i think thats just like a high whistleling.
I hope you know what i mean.

The high whistle is the Supercharger.
like xaotik said, I think it's high end transmission whine with bad looping.

Oh, ok

Yep, that sound sound is the gear whine.

Wait a minute, the FXR has a Turbocharger.