I had this problem, drove me nuts. I traced it in the end to where the brake servo fits, just below the windscreen. Plenty of WD40 in and around that area eventually cured it. Did this on the inside of the car as well as the under bonnet area. At the same time the bonnet itself was making horrendous creaking noises which again plenty of WD40 sprayed into the hinged section sorted. This all started not more than 10 miles after collecting from the garage when new. I repeated this process every 2 or 3 months until eventually after a couple of thousand miles it did disappear completely. Good Luck.
Sir P
Ps Don’t overdo the WD40 on the inside of the car, it drips on your trousers!
