Crepitus (in the knees) is a fine crunching noise, usually on bending the knee from standing.
Rice crispies?
Reminiscent of crushing a packet of crispie rice breakfast cereal, in the early stages crepitus can be felt, rather than heard, by laying the palm of the hand on the kneecap and bending the knees (while standing). The origin of the sound is apparently the presence of fine gaseous bubbles in the soft tissues, which arise from the soft tissues. Although the presence of crepitus is alarming, it just means that the knee is 'not happy' but is not diagnostic in its own right.
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eBook - Common Knee Symptoms
Easy to read downloadable book giving simple explanations of common knee symptoms.