Would you believe this...

The exhaust manifold gasket can also cause this noise and they deteriate very quickly anyway

Get a new one and replace it in any case, but regards to tapping...

Try depressing clutch fully, see if it deisaperas - clutch release bearing!

Change oil to Mobil Motorsport 15w40 - use new filter too, ideally at 4,000 miles intervals

Do not however get new hydrolic lifters (tappets) and use on an old camshaft - don't do no good for the engine - and it will only get noiseyer!!

All of the above worth checking