Sounds like a tappet noise

[mowogg]

I have just fitted a new engine to my car (1098 10cc engine with 256 cam, HIF38, MG metro inlet and 12g295 head)

After a few issues it now seems to be running reasonably well apart from an annoying tappet type noise which i don't believe is tappets. I replaced the rocker shaft, and cam followers on engine reassembly (along with new rings, mains and big ends).
It seems to be louder in the car, than listening in the engine bay, and is present with both the transmission enganged and disenganged. Listening under the engine bay it sounds like it is coming from the back of the engine bay (not water pumps/dynamo area. Compression is good (165 -175PSI)

Any ideas as to what might be the source of he sound?

[bmcecosse]

You seem to have covered most angles - I can only suggest an exhaust blow - perhaps at the head/manifold join - or piston slap......