AEX 8 valve engine problem

[Rmachines]

Hi, this is not a new problem, i have had this problem since i had the car and done a good 15000 miles on it, it made a knocking noise on acceleration which is very loud when cold and gets quieter/silent when warm, but its got worse latley although it does get quiter as it warms up, its still knocking. It is pretty much silent on deceleration unless in a close space then i can hear it a bit.

Its a clattery knock i get from the top end, at tick over its arround 3/4 knocks per seccond. Initialy i thought it was hydraluic lifters, infact i still do think that but it doesnt make sence!

1. i have never known a lifter to knock like this, only tick/tap
2. I have given them all a good poke and they seem to be pumping up ok
3. in 15,000 miles like this you would think it would have cause wear marks on the surface of the lifter/cam but i cant see anything unusual.

If i listen very closly, it sounds almost as if it comes from the exhaust manifold area, Its not that obviously!! it comes from quite deep within the head.
Theres only one thing else i can think of... and thats piston slap?? but its bloody bad slap if it is lol.
A realy need some advice on this as its anoying me and i dont want anyhting to suddenly go! the engines held up fine with it so its not anything like a big end. (although sounds a little like it)

cheers,