My car is a recently acquired 2008 Lexus iS250L. It has an unusual fault which is quite alarming. At completely random intervals there is a loud cracking noise which seems to come from the windscreen. It is EXACTLY like the noise when a stone impacts the windscreen whilst fast driving, although it can be louder.

I do not believe it is caused by stone impacts as
(a) it occurs too often
(b) it can happen when the car is stationary and
(c) it can occur on super smooth road surfaces.

It usually seems to come from the base of the screen although this is by no means always the case. It also seems to be much more prevalent when the heater air flow is directed at the screen. It occurs only rarely when the airflow is not directed at the screen. I have the air-con on all the time even in this cold weather as the car seems to be prone to misting whether it is on recirculate or not.

A previous car 12 years ago (a Peugeot 306 with air-con) had the same problem which I recall was remedied by reseating the windscreen although it’s too long ago to remember the details.