Your bike is pretty new, and the most wear to the sprockets and timing chain occur in the first few thousand miles. The plunger has teeth along it's length (the ratchet mechanism mentioned) and as the initial wear to the chain and sprockets occurs, at some point the tensioner will be between being loose on one tooth, but not yet able to click to the next tooth. So there is that slack in the chain when the engine is first started, but before the oil pressure can press the tensioner against the camchain guide, and take up the slack.
Long story short: Eventually the chain and sprockets will wear just enough for the tensioner to click to the next tooth, and the momentary rattle before oil pressure builds, will cease. At that point the chain and sprockets will wear at a very slow rate from then on.
Sometimes the tensioners can go bad, but that's rare. So I wouldn't worry about it.