I know this sound has been brought up many times, but from what I read mine is diff then everyone else's. I have a 05 1000 with a Yoshi 2 into 1 and K&N filter. I am not running a power commander but I dynoed the bike and everything is still OK, I gained 6 hp with the mods. My bike has done this to me twice. The other day I went to pull off from a red light and it made loud clunk sound, and now when I get on it around 5k RPM it makes a whistling sound. Leaving the red light the other day the bike back fired through the injectors to the point that you could hear and feel the flames hitting the bottum of that tank/air box. After it did that the bike quit whistling. Well last night leaving a red light it made the clunking sound again and now it whistles around 5k RPM again. Has anyone heard of this.