I've heard of and seen speedo cables buzz and rattle before. They can wear down on one end or the other, and they will rattle (bounce back and forth) on the instrument cluster end. Check both ends to see if they are tight, and pull off and look close for signs of wear on the end of the inner cable. Replace if necessary. Especially since if left that way, it can and will hammer a hole right through the speedometer. Picture that rare event happening. I would have thought your bike had electronic speedo, not a cable. In that case it's maybe a part or piece inside the instrument cluster, and will have to come off and be found and fixed. Don't know about yours, but mine can be disassembled, and individual parts are available. I like that. If it will do this when parked, try using a tube of some kind, to listen through. It will point pretty close to the location of the noise. Try from different angles and see what you get. Instrument clusters have rubber bushings or washers to mount with. Check and make sure they are all there, and in good condition. Headlight cases have a habit of doing that with wires inside. Lining with thin foam fixes it, the non-absorbent kind of foam. I use the thin foam that comes on new LCD monitors. But that is more of a buzzing than a rattle.