someone missed a bolt somewhere.......
Had this on my little Holden Barina. Someone had left out a bolt holding down the camshaft. I only happened to pick this up as I decided to check valve clearances at around the 40,000 - 50,000 kilometre mark.
So much for clearances having been checked while it was being serviced while under the 20,000k warranty!
I whacked in a bolt I happened to have laying around and never ever had an issue. The engine did 420,000 trouble free k's up until I disposed of it so obviously there was no ill effects....