Hey there , what you may find, as i was saying before with the lifters, they do become clogged,(as they are so small and have such small oil feed orifices) and sometimes hot oil and filter changes can fix this, when the lifters are noisey,after a while they can become internally siezed due to the engine sludge residue going hard inside the lifter mechanisms, as a result the hydraulic lifters turn solid, by this i mean they block up and stay literally dont expand and contract as per a normal hydraulic lifter operation, as a result when they sieze, they hammer the top of the valve stem, (in some extreme situations-or if left long enough) and as such mushroom or round the top of the valve stem,(which is the tapping noise you are hearing) now to fix this problem usually a replacement lifter will do this, but while you have the lifter/rocker assy removed, inspect the tops of the valves, particularly the noisey ones, and check for rounding or mushrooming, this will be visible by a highly glassed or glazed surface that is literally hammered or pitted and potentially blue in colour from heat. Also the base of the lifter is also the same, you can sometimes get away with just putting in a new lifter or lifters, but , if the noise still remains the only way to rectify the problem is to have the tops of the valves machined flat, so as to provide a flat surface for the lifter to ride on, in addition to this, with this mushroomed condition , valve rotation can also be hampered-(essential for valve cooling).
( But as this is also a case of strip down and visually inspect i can only advise as to the processes and their possible plans of attack, to rectify the problem and feel that its important to cover extreme cases, so as you are fully armed with information. )
If this is the case the only way to machine valve tops is removing the cylinder heads, and having a machine shop do this for you, but thats a way off right now, your best plan of action for the moment is to seek out the noisey lifter/s and have them replaced, you would possibly be better off to replace all of the lifters in one go, so as not to have to do this job again, -by doing this see how the noise goes, IF you find that the noise is still there, OR if you find that the valves are heavily hammered/mushroomed, when you have the rocker gear removed then the next step would be to either remove the heads, and have the valve tips machined, (also dependent on monetry budget) or try a new set of lifters and hope that it goes away- when installing new lifters, lube the tips of the valves with fresh oil so the lifters have something to ride/slip on and cushion the noise.
Also its best to install them dry-although you can soak them over night, but by installing dry-then idling the motor they wiil beed up -but will be a bit tappety untill they bleed up, then silence will occur once oil has reached them . (There wont be anything wrong with the cams, its the valve tops that will be the issue.)
As for the Timing Belt, i would recommend leaving this for a pro- however, only if you have experience in this area, and with compressing hydraulic tensioners-correctly- as there is a technique to it them, in addition with your engine if you are only 1/2 of a tooth out on the camshaft timing you can bend all 24 valves. From there you life becomes that much more difficult-thats why i suggest leaving it to the pro's-if they stuff it up they pay for it, and not you, plus theyre work is warranted, so you can go back to them with any worries, as a result of the work done and they have to fix it- and also finally, its just a huge job .
As for the wet/dry compression test- carry out a standard compression test on all cylinders,and record all readings, then do it all again however this time using an oil can deposit 2-3 drops of oil into each cylinder, as you go then record the results agiain.
If the compression goes up, which it should you have a perfect seal on compression, if it stays the same on that cylinder then you have a leaking engine valve- you see by adding the oil to the cylinder you are sealing the rings, so the only thing left unsealed is the valves, and if they are not sealing correctly then there wont be an increase in compression, but try the lifters first to see how you go then go from there!!
If you have any other worries , dont hesitate to ask!!
Best Wishes; Matthew (aka; matt0302)