Can anyone tell me what the cracking sound that appears to come from the wndscreen?, almost sounds like a stone hitting the screen.

I'll bet either your heat was up HI or the A/C was maybe on cool down when this 'CRACK' occurred it appears to be expansion and cotraction of materials with a quick vaiance in cabin temperature.

Happens to me BUT ONLY when heat turned up or quick cool using A/C

Maybe others will have other reasons

Early cars that suffered with ill-fitting Dash Locating Pins were said to occasionally crack & I do know that judicious packing with H.D. foam could cured it.
Tel

Sounds like that could be the case. i will keep an eye on the A/c and the temp.
cheers for that.

Just had mine in for the same, garage did a dash mod?? anyway now all sorted

They also put sponge between the face grilles and the radio/heater control and that has also cured the crack from there.

Simple but effective

Im guessing this was a Lex garage? and under warrenty? and without wishing to state the obvious this was done internally, yes?
And how long did this job take?

cheers tash

Yes at Lexus Newcastle, took couple of hours and they had the radio unit out to do it

Both my IS's have done this. Usually after 20-30 mins of driving. I have also assumed that it's due to temperature change, although I don't think it is limited to very hot or cold settings as it always happens eventually on any journey. I just don't worry about it since it only happens once per trip.