Mainly the sounds of fluids in the joints squishing around like a packet of ketchup.
The joints normally are seperated by the fluid and have a membrane around to help keep all the goop in. When you start really flexing your joint, it can squeeze down in one part, you roll the joint over and the pressure builds up and starts squeezing past the bone to bone interface causing the sound you hear.
Now if you are hearing crunchy popping noises, those are usually bubbles trapped in the fluid that are forced into compression and expansion as they flow through the interface.
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