engine tap/knock 3.9v8

Hi people,I am looking for some help with noisy engine.Its a v8 3.9 disco 1993 fitted with new cylinder block less than 15000 miles ago
its been built with, new cam and followers new rings new oil pump new shells new timing chain and gears, the crank was unmarked and well within limits and the pistons were all ok.The heads were stripped and nothing found wrong other than valve lap.Its been fine until about a month back I noticed a feint knocking coming from the engine after a fairly fast run It started at about 2000rpm and went at 2500.I have since towed 1.5ton caravan through derbyshire and its getting worse.Its more noticible under loading and when engine is hot Now for the bit that throws me If I drop into 3rd and let the engine rev the noise goes and its not there for some while after changing up again,then it gradually comes back.Its definately not big end Ive just had the sump off everything was fine I have replaced the shells though as I dont like putting back used ones for what they cost.Its not piston slap or little end. I can now get the engine to make the noise when its hot on about 1000rpm sounds like something is loose or broken possibly on the nearside bank.I think it could be a sticky/faulty tappet but I have never heard one thats failed so I dont Know for sure Someone has suggested a broken valve spring again I have never had one so I dont know what that would be like. The engine doesnt lack power and has good oil pressure.I have checked all of the obvious like loose exhausts,broken mouts etc.Given the wealth of V8 knowledge thats out there I was hoping somebody may have had something similar and could point me in the right direction any help would be appreciated Thanks for taking the time to read Alan
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snipped-for-privacy@sky.com wrote:

Hi, it could be a tappet, or it could be a loose liner in the cylinders? It could also be a loose or cracked exhuast manifold. They can make some noises that sound like tappets etc.
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Nige, 'Sing us a song, a song to keep us warm'

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Nige wrote:

I think that you may be on the right lines with a sticking/faulty tappet even though they are said to be new. Higher revs and colder oil giving relief fit that argument with the tappet failing to 'pump up'.
The effect is to increase the rocker clearance and you may be able to detect that when running with the relevant rocker cover removed. Try listening around with a stick/screwdriver in your ear to help isolate the area involved
I had a tappet where the non-return valve was a steel disc. The disc broke and the tappet would not pump up. There was about 6mm of rocker clearance and the noise was horrendous.
Don't rule out an exhaust leak, either.
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Have you checked your camshaft end float. I had a knock after fitting a new non genuine timing chain and gears. The gears were wider than the original and wore a trench into the front cover and oil pump. Also forced the cam to wear into the engine block giving me 35thou end float. Would knock up to about 2500 rpm and then go away.
Did you fit a "Rolla" brand chain and gear kit?
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Chas

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