Very noisy windows

[wawa]

Got my 2000 boxster S in april. Love it so far. Lately temps has finaly been higher in Quebec City and have use the top a lot. I have also use the windows and find out that when its hot and dry outside, they make a very loud squiky noise. It is as disturbing as finger nails on a black board. What can I do about that? Would sliding product on the seal help ( like gigoloo) or it will leaves mark on the windows every time I use it? Any other suggestion?

thanks
wawa