The windshield defogger air is blown through the gap between the dash and the windshield. If you fill the whole gap with foam or felt, you will likely cure the clicking noise, but will get absolutely no defogging! It's best to fit four short pieces of material into this gap, two adjacent to the tweeter grilles, and two crossing the junctions between the dash proper and the removable central "cover" over the clock. That seems to have worked for me so far.

Stan