Hi all, I've noticed a rattling sound from what I think is the left hand side of crankcase, while I'm coasting in gear it's noticeable, and when I press the clutch lever it all but goes away, I'm guessing main chain pinioon loose on engine? But don't know. Any help would be greatly appreciated.
TIA

Check the cheap, easy and obvious first like the condition of the chain and sprockets and the chain's tension. Then have a look for any play in the drive line whilst your at it. Worn sprockets, particularly the gearbox one, tend to make strange rattles under coasting or engine braking as the wear pattern they develop matches the forces and how everything beds in under power - the gearbox sprocket is driving, rather than being driven, if that makes sense.

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Thanks Plaiedes, firstly I apologise for not having renewed my subscription- I now have (hadn't realised it had lapsed).
It's worth every penny for the continued support.
Yes, I know the chain is slack, the bike is now 5 years old but I keep it cleaned and clean the chain with WD40 chain cleaner and use castrol racing chain wax, but have never adjusted chain other than mechanics changing tyres. I will look at that forthwith and publish my results.
I take it that with the same sprockets for the 5 years, I ought to replace the both with a new chain now (i don't stretch the chain with super rapid acceleration)?

Actually, there's one thing that puzzles me-perhaps you have an answer for this.
Given that the main drive sprocket is turning all the time whilst the chain is going round, why should it make more noise when coasting in gear than with clutch pulled in, or for that matter, when coasting out of gear (during which time that main sprocket is still in play)?
Haven't had the time to look at the sprocket to see if it's loose yet, but will over the weekend.

What you have to consider is that the gearbox sprocket when it wears will tend to ‘hook’ the teeth as it pulls on the top run of chain; the chain’s rollers naturally get drawn into the well between the sprocket teeth when 'under power'. This is where 95% of chain/sprocket wear comes from as for the vast majority of time the bike is moving it is under power and the gearbox sprocket is pulling the chain in this way.

When coasting, on the overrun, using engine braking etc. (whether in gear, neutral, clutch in or clutch out) the gearbox sprocket is being driven by the rear wheel - the forces are reversed. The top run of chain goes slack and no the bottom run is taught because the chain is now pushing the gearbox sprocket round (not being pulled) – the forces are acting the opposite way round. The rollers of the chain are now being pushed against the opposite faces of the teeth that would be taking the strain under drive. What tends to happen is that the chain rollers pushing on the sprocket want to slide/ride up the sprocket teeth to where the gap between teeth is largest which is what makes the noise you hear, the rattle or grumble. (Under drive remember that the rollers want to get pulled tightly into the well between the teeth where there is less play.)

The reason, when off throttle or coasting, there is a difference in sound between the bike being in gear and the clutch being pulled is that the ‘pushing’ chain on the gearbox sprocket meets much more resistance when in gear and clutch is engaged (fighting against compression, flywheel, internal friction or meshing gears etc.). When you pull the clutch you isolate the gears, flywheel, compression etc. so the gearbox sprocket is much more easily turned by the force acting on the rear wheel and pushing/driving the chain round. (Coasting in neutral is somewhere in between – the gears are still meshing but flywheel mass and compression are taken out of the equation.) As a result of the lower resistance with a pulled clutch, the chain rollers don’t want to ride up the sprocket teeth as much or as far as when in gear, so sit lower and tighter in the well between the sprocket teeth where there is less play and less noise.

All chain and sprocket combinations make a different sound under drive from when on the overrun even when new. The noise difference is less noticeable on a new C&S kit though because the leading and trailing edges of the teeth are the same shape and symmetrical. When the chain and sprockets wear, the teeth become asymmetric and consequently the sound difference between load/no load becomes more pronounced.

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Many thanks Plaeides, and very sorry for late reply, family problems took precedence.
For those following this thread, it makes perfect sense, and tightening chain to correct tension mitigated this effect, but not totally.
#Is ther C&s chainset the way to go? If so, whose the best supplier? many thanks once again.