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Bombing along all is fine, close throttle to change up (or down) then get a brief rattle. If throttle off to slow down in same gear rattle stays for longer as it coasts. Throttle back on / revs back up and rattle stops. As a comparison, it sounds like a loose cowling (but cowlings are ok)

After a bit of google diagnosis , and sticking my head underneath while revving it to have a listen I'm thinking exhaust inners. Standard exhaust couple of years old if that, 3,500 - 4,000 miles.

When I say "bombing along" I mean in a standard 150 kinda way. Otherwise running ok. New piston rings prior to Shipston so just finishing running in, about 500 miles.

Do these symptons and my exhaust theory tie up?

Many thanks as usual.

It does seem you could be right. If you take the exhaust off and shake any loose bits out you may save yourself some grief later on. The bits if they get small enough to enter the tailpipe can lead to a blocked tailpipe. This in turn will lead to a serious loss of power followed eventually if you ride far enough, to a holed piston.

Pete

Take it off and shake any bits out of the big end. An exhaust is classed as a disposable a mate has just had the same with an MB one and thought 10 months and Euro trip about right. Personally I would not for the price.
He did say he had only used Barbie paint but if they fall to bits that quick I wouldn't bother.

I had a mystery rattle once. I suspected cowlings and maybe silencer but it turned out to be a loose flywheel fan. You couldn't feel it when the engine was cold but warmed up it was sufficient for the metal expansion to loosen it enough to rattle the flywheel cowl.

Paul