If you use silicone spray etc, get the spray can with the long nozzle that can be used to direct it behind the hard plastic gasket, that fits between the windscreen wiper scuttle and the bottom of the wind screen glass.
Put the wiper arms / blades in the service position while you carry out the work, you can use some glass cleaner / APA / Meth's to remove any small amount of over spray silicone that is on the very edge of the gasket / glass.
The wipers blades don't reach that low on the glass, so little chance of them becoming affected.
Don't worry about the gasket, it is not really intended to be a fully water tight weather seal.
It's just a plastic finishing gasket that is intended to provide a neat finish between the back edge of the metal wiper scuttle panel and the mating surface of the front wind screen glass.
If you look at the top of the scuttle panel, there is two long sets of vents that allow air flow into the car via plastic duct work.
Water is going to enter through these vent holes in the scuttle panel, but it is forward of the main bulkhead of the car and are designed not to let water into the cabin area, water is channeled away on to the ground.
There can be various levels / methods used to help solve this annoying "Cracking / Creaking" noise, which appears to be coming from the front of the very front of the dash panel, but on the inside of the car.
But the source of the problem is outside of the car really.
A couple of fix's used with good results.
Simple method :- Easing back the plastic seal with a plastic trim tool or even a old plastic bank card etc. Then in sections and working across the length of the gasket, spraying in some type of silicone spray etc down behind the gasket using a long nozzle on the can.
Or go a little further :- By removing the clips / screws that secure the wiper scuttle panel to the bulk head, which will allow the scuttle panel more movement, then access is improved to apply a suitable lub and then replace all the fastening screws.
Full on job IMO :- Firstly, use two pieces of masking tape on the glass to help return the arms to the correct location when refitting, then remove wiper arms / blades from the car ( 13mm socket if I remember correctly and you may need a simple puller to do this job ).
Then completely remove all of the scuttle panel screws / clips and remove the panel from the car.
Lay the panel upside down of a flat surface, covered in large towel or better still a sheet.
Completely clean the plastic gasket and the bottom of the windscreen with Meth's / APA.
Now using a length of self adhesive "silent tape" fix it to back of the plastic gasket, completely from one side to the other.
Refit the scuttle panel / screws / clips to the car and return the wipers to the normal rest condition and refit the wipers arms and tighten to spec.
The silent tape has a soft felt like surface, that it prevents the hard plastic gasket causing a hard foul condition against the glass.
The cracking and creaking noise is caused when the body flexes, and the hard gasket does not want to move freely across the glass, when it does so, the noise is produced.
Of course changes in ambient temp conditions affect it, the noise is more noticeable when it is cold I have found because the plastic gasket becomes rigid due to the conditions.
Incorrect choice of material for the gasket has produced this issue IMO.
A softer silicone based gasket would been a much better option.
