Twitching / "tjirp" sound

[Ramius]

Lately noticed a "tjirp" sound when the pig is cold and "blipping" the throttle. (on the first split second while the revs pick up). Sound very much like a fan belt slipping. Yes, I know.. can be the alternator belt.. but I've checked the tension and its quite tight (can barely twist it 90deg).

Note that this is only when the bike is cold. any ideas?

Thanks

 

[Welsh]

 

[Beebop]

 

[katana]