There is no doubt my chirp/bird/whatever is the speedo.  It could easily be that the cable need be lubricated (I used to have to do that on my bass boat with graphite lube).  Anyway, on mine it has NOTHING to do with the weather...if the bike is moving under power, I have a chirp of some sort or other.  

I don't hear it as much since I installed the cool-sounding-but-very-loud Screaming Eagle muffler...I foresee a session with the tank off the bike in my immediate future...replacing a speedo cable, maybe a speedo housing gasket, fixing a weepy oil plug, replacing spark plug, checking carb for jet size, etc.