st fiesta top end tapping

[ch1975]

my fiesta st 55 plate 40000 miles has developed a tapping at the top end from 2000 rpm up, and was getting lazy to start (turning over about 5 seconds before firing. did a compression test and found pot 1 only had 8bar, 2 had 11bar, 3 and 4 both had 12 bar. removed head today expecting to find a hole in piston or a valve problem but all looks ok, waiting for head coming back from testing (vacuum and pressure and a skim), has anyone got any idea?

did a sniff test before strippig all ok.

head gasket looks ok

cams are both ok, (will need a polish on caps) but no pitting or gouges just discolouring all followers look ok.

next step is bottom end off but it was deffinitly a top end tap