Never heard a sound like that coming from one of these

@uber_beluga absolutely no work has been done unfortunately. The owner has no disposable income at the moment

@calum I reckon he might be wanting to clean it or replace worn parts

Is that the original colour of seat cover? Very nice result

Unlucky man, glad your unharmed though!

This bike in the UK?

@Nottsbiker is there many pages? It would be great if we can get them uploaded into a thread of their own.

Just had a quick Google and first couple of results suggest worn rings / not correctly sized piston, which makes perfect sense because the sound will be the air squeezing through the ring seal when compression is happening.

I suppose this means my bikes could have been a bit down on power not having a correctly sized piston

A couple of my bikes used to occasionally make that sound when I was kicking them. I'm sure it only happens on an engine that isn't perfectly set up / running little bit lean or little bit rich? Either way the bikes ran okay.

Very nice man. You've got plenty of room on that Speedo bracket, you could fit a few more bits I've always wanted to fit a clock but haven't got round to it yet!