He's suggesting that the bearings on the belt idlers are worn and that the
balls aren't quite round any more. When the engine is cold and the grease
is caked up, the balls rattle around. When things warm up and the grease
softens, it's okay.
There are two ways to tell if this is the problem.... first is to take
some 5mm fuel hose, put one end in your ear and wave the other end around
in the engine compartment. When you get the end close to the noise source
you'll know it, and you'll be able to tell if it is coming from the chain
cover, the valve cover, or an idler.
The second way is just to squirt some aerosol grease into the bearings
on all the idlers and see if they quiet down.
Do the rubber hose trick and verify that it really is coming from the
timing chain cover. Then pull the cover and look inside.
"C'est un Nagra. C'est suisse, et tres, tres precis."
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