Engine Knock???

Over and above the normal 800GS engine noise I have now picked up a definate "not normal" noise! I first noticed it about 2000km ago. Just above Idle hot or cold,clutch in or out there is a distinct da da knock. It is not adubible anywhere else in the rev range. Any body else come across this? It is very distictive and you can feel it resonating through the engine case when it happens. Bike going in for 20k service this week so will get the guys to check it out. Almost as if something is unbalanced? Worrying me a little to say the least