Also - while you are in the filter box, look for leaves! They seem to make their way in there to wither and die.
Also - if you do have leaves in there, dont try and pick them out by hand - as they are probably the most dry and brittle leaves you have ever seen, and will instantly dissintegrate into lots of small pieces, leaving you with a right mess, and lots of dead foliage just waiting to be sucked up into the engine if not removed properly.
Just remove the air filter and suck out any invading leaves with a hoover or similar.