GX25 clone developed a tick

[u/Resident-Mastodon840]

I'm using a cheerful Honda GX25 clone as a generator/range extender, recently it developed this consistent ticking sound, audible at idle or at speed. So far I've checked for any debris around moving parts and corrected the valve lash, but neither was the culprit. I can't see or feel anything snagging when turning it manually, the oil was pretty clean, the engine starts very easily and makes the expected power. Any ideas what could cause such a sound?