Hey - I did the roll your own airfilter out of a furnace filter a good while back. Last week I started to hear a high pitched whine from under the dash... thought it was the blower.Turns out the filter had kinda deteriorated & got pulled down in there... the metal wires were rusted to the point that some pieces were missing.Now that I've removed it the whine went away & I'm hoping the metal pieces didn't damage the fan.Anyway, if you go the furnace filter route, replace them often!