Recently had a full engine rebuild - 4.2L on a S1.5. Not yet street driven. Engine runs smooth, cool, with good oil pressure. The engine has a rattle - not very loud but noticeable - which is only there at idle or not under load. If I rev the engine, it disappears, and only returns after revs drop back down to idle. It doesn’t sound like a high tick-tick-tick of tappets, but I may be wrong. I checked the tappet gaps on the exhaust side (I was concerned the guides were moving) and they were all about 0.008. I can see there is slack in the chain that runs from the inlet cam sprocket to the central tensioner back up to the exhaust cam sprocket. I have turned the adjuster CCW and taken up a bit of slack. It feels like the adjuster is at the end of its range. It’s meant to be a hand adjustment so I’d rather not use any tool to turn it harder. Is this noise likely to be the upper chain or tappets/guides?
Are you sure it’s the engine? Could be the reverse idler in the gearbox. Does it go away if you press the clutch pedal?
It’s clearly top end. If I remove the oil filler cap it becomes louder.
Chains can slacken slightly after fitment due to squeezing the factory grease out between rollers and pins. Once you are driving the car you will be having too much fun to care about idle noises. The chain adjuster eccentric works either direction provided you go far enough. Better slightly slack than over tight.
Tighten the upper chain. That will provide you with an answer.
Personally I disagree with Peter. I think the tighter the better.
Having said this he’s a lot older and more experienced than me.
And probably better looking.
I think you’re being very hard on yourself Andrew 
- clearly a typical self-deprecating Aussie. Everyone has given me the confidence (plus older posts here) to tighten up the upper chain tension a little more. I turned the eccentric CW rather than ACW (should effectively be the same if’s a symmetrical eccentric?). I can still push the chain on each side of the eccentric 8 or 10mm with a screw driver. Anyway - I fired it up and it appears that the annoying rattle has gone. Now to the tappety tick-tick-tick when revving my XJ6! It’s only noticeable when the window’s are down - another post Thanks all.
Chain tensioner released and functioning?