both tensioners rely on a good supply of high pressure oil through tiny orifice in the block which hydraulically pushes the sliding shoe outwards against the timing chain to apply enough tension that stops it flapping about.
if at any point the oil gallery were blocked by thick sludge and/or unfiltered foreign objects from lack of service (very old abused oil, clogged oil filter allowing unfiltered pressurised sludge to bypass the sprung relief valve and straight into the tiny oil galleries), oil pressure can't get to the tensioners so they remain completely unloaded against the fast spinning chain. this means the loose chain and the tensioners can just rattle/flap about uncontrolled and creating alot of noise, probably more wear/tear and more risk of skipping a teeth.
the upper tensioner features a progressively stepped stopper ratchet mechanism which is suppose to mechanically stop the sliding tensioner shoe from being pushed back afew mm from where it was resting but there's a limit to how far the tensioner can be extended before the ratchet stops working.
the lower tensioner is simply a lightly sprung sliding shoe without the stepped ratchet stop and relies on oil pressure. it has a limiting notch to prevent it sliding out too far.
if the chain is so severely stretched that the tensioners reached their travel limit, unable to apply anymore tension and making it rattle, it's time to either replace/rebuild the chain or swap engine.
a slight chain rattle for the first few secs during cold start is normal cos there's not enough oil pressure yet.
after bout 5sec the oil light should be off and it should get quieter.
if it still rattles badly then either there's no pressure reaching the tensioners or the chain is really stretched.
I suggest removing the side timing chain cover to see how far the upper tensioner is extended and if it feels really loose/sloppy.
- if its not fully extended and it feels snug against the chain then it could be lack of oil pressure reaching the tensioner.
remove, disassemble and overhaul/flush the upper tensioner assembly clean. check if the face of the tensioner shoe ain't worn down to metal. then poke clear the oil hole in the block with a pin to unblock it from sludge. reassemble, new oil & filter, prime oil system then start her up.
- if its fully extended and/or feels loose/sloppy, the chain could be worn and needs replacing or engine swapping.
