Well, I've had some success.
Ween I was looking the engine over, and generally swearing, I noticed that the gasket for the oil filler looked somewhat crap. A quick look around the net and local auto parts, and a gasket for the oil filler couldn't be had (unless I bought a whole gasket kit, and waited for it to come from the US).
So off I went to the local auto store to gather some supplies. New bottle of 15w/40 Nulon oil, Nulon Oil Flush, a new sump plug (the old one, while only a couple of years old, put up a valiant struggle. I had to take a metal file to it whist still on the sump because it was jammed hard and was rounding), some gasket stripper, and some gasket paper. I also bought some high temp silicone as I couldn't find my other tube. Of course, I found the other tube as soon as I got back. I'll just keep the new tube as it may come in handy.
Unbolted the oil filler. I then got a small section of rag and stopped the entry into the engine so nothing would fall in! Then I hit the face on the engine block for the oil filler with the gasket stripper. I let that set for 5 to 10 minutes.
I then traced the outline of the oil filler onto the gasket paper and cut it out. I used some radio nose pliers to help 'trace' the bolt holes and the throat of the oil filler. Then I cut that out. One new gasket!
First time I made one, and it came out great, at least I think so. Of course, I forgot to take photos.
I know I could have used something else other than gasket paper, but it only cot around $10. I dimly remember dad using cardboard from a cereal packet. But I'm pretty sure he only did that until he could get a proper gasket.
I then very carefully removed the old gasket using a safety razor in a scraper handle. Making sure none fell on top of the rag. If any did, I made sure I got it off straight away. I also made sure not to score the face of the engine block under the old gasket. I then did the same to the face on the oil filler. Most of the old gasket was on the engine side of things.
I then smeared just a little of the high temp silicone onto the oil filler face, like about 1mm thin. Then I stuck the new gasket to the oil filler, then put more silicone on the other side of the gasket. Then I bolted it home.
I let it sit for a bit, then I turned the car on. The tapping noise hadn't gone away over night. Shame. I put in some of the Oil Flush, and let it run for 15 minutes. Then I turned the car off and let it sit for a few minutes. But not too long as it's best to do it while the oil is still warm. Had fun trying to remove the sump plug and not get hot oil over my hands. Had a partial success. lol Let it drain until I couldn't see any dripping out. I also had taken the oil filler car off and the old oil filter (really only a couple months since it was fitted). Fitted the new oil filter, and the new sum plug with the old washer. The new ones are a white nylon. My old one is copper. I prefer the idea of the copper one to the nylon. The composition of the nylon is probably a high temp, but I know the copper one won't melt. lol
Put in 4 liters of the 15w/40 and went for a spin. Just worked it out, it was around 90klm and took about 1.5 hours. give or take. Mostly crusing at the speed limit. Had a, erm, spirited competition with a Bentley GT, and lost. Had another spirited competition with a Honda Prelude, and won. Well, at least until we went our separate ways.
About half way through my drive, I noticed the tapping had stopped! But it wasn't to last. It's now somewhat intermittent, but is much less noticeable most of the time. So now I'm leaning towards a sticky lifter.
I was going to buy some Lifter Free additive (also Nulon), when I went to the auto store. But I was hoping this afternoons work would fix my problem.