Odd noise

[Steve kay]

Have you lost a nut in there- Peter

That was my initial suspicion, which is why Iturned the engine upside down, in the hope that a lost nut might fall out, or be helped out by turning the engine and thus flywheel. However nothing has come out. That might be total sod's law, something drops in but won't drop out. Still puzzling if the noise only occurs when the engine is "right way up" not at other positions.