[MBEngineering]

HI Monkeydub
As the 3.0Lt Carrera engine has a lot of issues from new, the noise you have is poss' valve train related, and some one has fitted the pressure feed tensioners to try to sort a larger prob'.
you need to check engine history back to day one, as the engine should have been back to the dealer under warranty for a full engine strip/rebuild with the incorrect valve spring shimming, which needed to be corrected, back in 77/78;

the job's required to be sorted; full engine strip.
head strip and new valve exhaust guides.
new bearings.
new inter-gear, chains and valve springs,
modifi the crankcases and tin work,
replace head studs.
this lot should have been corrected, and I say "should have been", I still come across Carrera engines even now that have not had the work been done, thay are now in a bad state of disrepair.
is the car in the UK or state side?? if it is UK based and any where near me I do not mind having a look for you to asses the noise.

regards mike