Had exactly the same problem which was doing my head in. I ended up stripping the engine completely to no conclusion..BUT it is now cured.. Very simple after all that.. Changed oil, filter to high grade 10/40 and made a oil filler breather. The only reason I came to this check was from time to time I noticed the bike had more torque than before and the clutch would become stiff and heavy. So after no end of research and came to my own conclusions I ended up thinking there may possibly be too much air building up in the crankcase and causing loads of torque to release the air pressure because after a short blast the torque would slacken off and would get a rattle..
Now just to point out my fazer 600 98 model has a few performance upgrades which may not of helped with mine but after I've made a oil breather to replace the filler cap, blocked off the air inlet to the air box and rerouted the crankcase breather pipe, all rattling has gone and even the fuel economy is better, and throttle response is 100% better. Hope that gives you some more insight into a cure for you..
interesting! would be worth doing a little how to on this or at the least a few pics please
Will Upload pics Later. A little how to.
I managed to find a valve stem at work with the same thread and size as my oil filler cap, removed the bearing from inside the valve as it was inlet and not outlet( would suck in but not blow out) which left the valve open.. Simply went shop bought a fuel hose(in red to match bike), air filter with stem on and just screwed one side of the pipe to the filler cap end and the other side to the filter and cable tied it along the frame and behind the pillion foot peg(you can smell the fumes coming out, so rather behind me).
Crankcase breather- removed the breather pipe and bought a longer peice. Had trouble getting the right diameter, so just soaked the biggest hose I could get in boiling water to relax the rubber and fitted snugly over the crankcase flute and then just routed it through the same restraints as the fuel overflow pipe down under the bike..
Air box.- Very simply just removed the L union and put a rubber blank gromit into the hole as didn't want extra air getting in. Does throw mixture slightly out but very easy to make the necessary adjustments to compensate...
Also removed the blank plate inside the airbox to allow the fresh air to circulate around the filter better.. Now sounds like a turbo under the tank as the air is sucked in lol. Everything seems to respond better with these very simple mods,. Throttle is more responsive, fuel economy is better, and the annoying tick is gone. Also power delivery is smoother..