hydraulic lifters are getting over 20 years old for most CAs, along with valve springs losing tension it could be just one Lifter sticking from build-up.

Wet compression test. Little bit of oil through spark plug hole. If comp comes up its rings. Otherwise valves id think.

Wet test?

it's been running for months without any major work .

I am thinking that when I connected the detonation sensor it has done something via the ECU ... maybe I had the timing too far advanced previously making up for the detonation sensor not working and then hooking it up without resetting everything has damaged something ...

I did run it for a while on I heard the noise

then again I could just be rambling on

Oh Michael ... yes listened for the knocking and heard it mainly in the top

Did you do a wet test too?
No loose nuts fell into inlet etc?

perhaps valve noise you mean?

have you listened to the motor by pressing a large screw driver in different areas with your ear on the back of the screwdriver? you should be able to tell if its coming from top end or bottom

so if I am lucky it might be a spring in the head on cyl 2 ... second guess

argh what a shame!

i dont think simply a bearing would drop compression

I think I have a big problem .... anyone out thee got any other ideas???

Compression tested 170 130 170 170 number 2 is 25% down (not good) Tested last week all at 170
Timing is ok about 13 degrees BTDC
disconnected one coil pack at a time - noise unchanged
checked with car under load - it got worse
no oil in water or water in oil
no codes on ECU (detonation code 34 now gone)

my guess is a bearing