i tried a gerni and it still wouldnt shift the crap built up under the cowl covers. so i took the drastic effort of removing the cowl covers to investigate. this requires removal of the wiper arms and also the strut brace so it's not a quick task.
cleaned up all the crud built up (leaf matter, dirt, sand, etc) and scraped it all off. I put a 10mm wide strip of self adhesive neoprene along the edge of the windscreen as a decoupler to prevent 2 hard surfaces hitting each other. noise has been gone since i did this
clark rubber had the neoprene but it was quite thick so replacing the cowl covers took a lot of effort.