I had the exact same thing with mine last year, started with a ticking then went to the same noise. I thought it was something in the chamber and was rattling around as the piston was hitting it.
Took the plug out and the manifold off. Made an attachment for the hoover that went in the plug thread and blew through with an air line at same time.
Seemed to be quieter after that only for the bottom end to let go a few days later
I would sort it out before you go riding about on it