I have a replay somewhere where the grasshopper is super loud. Was at Paul Ricard. I'll find it and post it here.
We managed to figure out what is making the sound. It's the Ghost Car emitting a distorted sound effect while waiting at the finish line. If you watch the video @WozzaUK posted, you can see how it passes him and turns onto the finish straight. Then after a little while (when it reaches the finish line) you start hearing the noise. It's been added to our bug database and will be fixed as soon as possible. Until then you can remove the sound by lowering the "Opponent Car" setting in the Sound settings or by disabling the Ghost Car.
Was just about to say i can't hear it unless @MeMotS killed said grashopper my oppents volume is at 70 and i hear no clicking ?
Funnily enough I was having the same feeling myself. I noticed that if I didn't do a complete lap there would be no ghost car and therefore no clicking. Thanks for looking at this for us.