Its the glide ring on the piston rubbing against the shock body. I would assume this is an older model monarch, 2011 or 2012. It was a common issue. It doesn't hurt anything, but can get annoying. If it bothers you, you can go to a local shop and have them contact rock shox to see if the will rebuild it with a newer shock body for you. Or contact suspension experts to see what they say.
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