Why does my motorcycle sound different (almost like a loud fan or compressed air)

hello, im new to motorcycles, and have NO understanding of mechanics (cars/bikes). I mean none. I couldnt tell you one part of the bike in terms of the engine, let a lone how to fix or diagnose a problem. Lately, ive noticed my ninja (which is a 2006 in PERFECT condition) has been sounding like a loud fan or compressed air is being released when i rev the engine. I dont remember this happening last summer. Ive owned the bike for about a year and MAYBE this is how its supposed to sound, but it just seems different. it almost sounds like hissing air and it is noticeable when the throttle is opened. I had the bike serviced at 500 miles. i did drop it once, and it barely caused ANY damage at all. perhaps this did some internal damage? how can I tell? It seems to run the same, and feels fine most of the time, it just sounds like its working harder because of the air sound. does anybody have any ideas as to what this could be? THANKS!!!