On my (cheap OEM style, very non-fantastic) bathroom vent fans,the screen underneath sometimes works its way up and interferes with the fan blade, either making a very loud scraping noise or stopping it altogether. The solution, of course, is to wiggle the screening down a shade until the blade is free to turn properly. I guess a more permanent solution would be to make some sort of a spacer to hold the screen away and attach it to the fan motor mounting brackets at an appropriate location...or install a more nearly fantastic fan.