Could it be the dampers? When you slow down or brake the weight will shift forward which will move the dampers. Does it also happen when accelerating?
Motorcycles
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I don't think I've heard it speeding up. Will try and notice.
Turns out it does happen when accelerating.
Seems someone else already suggested looking at the front end (though check headset bearings), so it might be a pivot related to rear suspension: check swingarm, suspension pivot points etc.
Does the sound change with changes in breaking pressure?
I don't think so. I can hear it with and without breaks being applied.
With the motorcycle off can you sit on it and bounce up and down to try to hear the sound and where it's coming from?
I cannot hear the sound; Just sounds like pushing air out of a bag or something. I cannot get the squeak to come out. Neither does rolling it on neutral while off.