Not a noise but a chirp. If I didn't know better I would say there was a bird under the seat somewhere. At certain intervals at cruising speed I heard a sharp chirp. It seems to be like a release of pressure or vacuum type thingy. It started last week and I can't located it while riding and it doesn't do it any other time. It may happen every 30 secs. or a minute etc. If I accelerate and back off the throttle I can hear it...I have had a lot of noises on this bike and I found them all but not this one. Any Ideas? and yes I did look for a bird nest.