I also am an excellent and skilled motorcycle rider…so much so I ended up with my very own TPF! hahaha
Anyways, I have exactly what you have. Straightening the knee sounds like someone stepping on a bag of potato chips. My physical therapists and doctor does not seem too concerned with this at the moment as it is common.
Since there is no pain (no pain for me, either), I have been told the most likely cause is fluid and air that comes with the breaking up of scar tissue and general healing itself.
The way I look at it is this: I almost lost my lower leg, so if dealing with some weird pain-free popping and cracking sound is what I am left to deal with, I consider myself lucky.