Kinda hard to tell the source from the brief video, but it doesn't sound like a happy sound, though these engines can be rather noisy of their own accord at certain times.. Could be a number of things, from badly adjusted valve lash, broken valve spring, part of a valve seat broken off, timing chain tensioner broken, or flying squirrels. Exhaust leaks can sound odd, too, as there is a split ring that is compressed against the exhaust port of the head under the spider nut, and it that is broken it could conceivable rattle with the piston strokes. At least the engine seemed to still fire on that side, so probably not a totally nonfunctional valve train.
First thing I'd check ('cause it is the easiest, and the sound could be related to the valve train) is to get the bike on the centerstand in the garage and remove the valve cover on that left cylinder, and inspect the rockers, valve stems, valve lash, springs.
Then, take the spark plugs out of both cylinders (leave the valve cover off - you can also remove the right side one to do comparisons). Remote the round rubber plug/bung from the leftside engine case so you can see the flywheel and timing markings. Use the clutch and get the gearbox into 3rd gear - you may have to do a bit of rear wheel turning to get the gearbox into 2nd, then 3rd gear - 3rd is much easier to turn the engine round. After this is accomplished, use the rear wheel to rotate until you see a small "T" (or S or Z) in the engine case hole... if you see a Z then the "T" will be coming up soon after in the rotation from the top->bottom in the hole. You want to get the "T" (there is a small line just above it) basically centered in the hole for valve clearance checks. When the "T" is in this position, both cylinders are at TopDeadCenter, but one side will be on the compression stroke (both valves closed and the pushrods should have no pressure on them, so you can rotate the pushrod ends with your fingers, and also observe a small wiggle-able gap between the top of the valve stems and the rockers that press on them. The other cylinder will be on exhaust stroke and will have pressure on the pushrods and valve open. You want to check for the proper clearance on the side at compression stroke. Rotating the wheel until the T comes round again will have the cylinder's states reversed, and you can check clearance on the other side for comparison.
You can also shine a light inside the cylinders' spark plug holes to see if you see any loose metal in the cylinder as the pistons move in & out.
Have you done this sort of work before? It isn't too difficult, just take your time and be very methodical and you won't do any harm/damage.
Report back what you see/take pictures.
