When I was young, foolish, and damn'd quick, me and my 850 Norton went to Holland, and I decided to run it around the Zandvoort circuit
I really gave it the beans... And then a horrible noise. Just like Greybeard's... I came into the pits gently, and the noise went away. Subsequent investigation back home revealed that when I had got it good and hot, a valve guide had come loose!!
The trouble is, that will be hard to find, When the head is hot, and the guide loose, it makes a hell of a racket, because it is moving within the head,
This could put a lot of strain on the followers, as they are now trying to ram the guide back in place?
I noticed someone said that the pipe was a little "blued" on one side. My GF pipes are only "light straw" at most. Check ignition timing, carb mixture, and if there is a problem leading to overheating, fix that first, and ride very gently. See if all is OK then.
If it is, I am guessing you will then have to have the head off, and check the guides. Check for evidence of one valve stem being cleaner along its length in a different position to other valves.
All the foregoing could be "brown smelly stuff" as a result of that being 45 years ago!!!!
Head experts, anyone?
My Norton? They gave me a new head, under warranty. Hope your head is not drastic.
Cheers,
Stan.