expensive sounds...

After the initial start-up of the OW01,
I haven't had time to get back down and see if I could get the bike running longer.
Tonight I went down to the bike and had another go.
This time, she started up and I managed to keep the bike running for a few minutes.
Everything seemed to be going well, but suddenly a really nasty sound was heard. A really high pitched screech, but it wasn't a constant.
I took the cam cover off and couldn't see any issues. But there is an issue for sure.

By all accounts I'm guessing it's going to be a proper issue....no idea what as head, but I reckon step one will be to whip the head off.

As I've never heard that sound previously, I'm clueless as to what the damage could be.
Facts are:
a) the sound wasn't there until after I kept her running for a few minutes.
b) the sound wasn't there any other time when turning over the engine- until she started and ran for a while.

it's only happening at a certain degree of when the engine is turning over...

Valve? Piston ring? spring? seized something?