Himm I don't have much more .
I've had these old thumpers for 26 years and never had this prob ,
Cam chain flogged out worn ?
Could it be a stuffed cam bearing ?
Could it be worn stuffed cam chain guides ?
Brent j on here maybe of some help ,
I remember him telling me how he had some sort of noise I think ?
He had to pull it down a few times to fix it but he told me this a few years back .
Or just take the baffles out of it mick so you can't here it
Or it's just a ticking time bomb due for a rebuild