Hope V4s are allowed in this section!!

When starting from cold the engine sounds perfectly quiet, but after about a 5-6 mile run or when the engine has reached full operating temperature a very loud knocking develops, but only on tick-over.
If you raise the revs slightly to about 1200-1500 revs - all goes quiet.

One of the many checks I have done was tappet clearances and all were good, but I decided to try putting a feeler gauge into each clearance whilst the engine is running and with the feeler gauge in no5 valve (inlet) the noise completely stops. Pull it out, itís knocking, put it in, itís quiet.

So I changed pushrod and rocker arm from no5 to no6, and the noise was still on no5.
So then I was thinking cam follower or camshaft, but Iíve just taken the head off and the followers out and all looks good. I also removed the cir-clip in the top of the follower and taken the seat out and all looks perfect.
I have also taken no5 valve out of the head and canít see a problem there either.

Iíve been a mechanic all my life but this one is beating me at the moment. I could fix it if I could just find out what it is, has anyone else had this problem.

Iíve uploaded a YouTube video of the noise for those that are curious here: https://youtu.be/mYYkgocVrRg