Replace chain, rails(?), and tensioner IMMEDIATELY.
Some say you can wait until you hear it, but you are risking bent valves or more.
100K Miles is the interval. Cost is $400-500. Depending on what other things you have done.
I've posted a note below from a fellow list member of his points.
>>>>>>>I have a 1988 300TE and I changed the timing-chain (and sprocket;
tensioner; upper cam-cover gasket; valve-cover gasket; cam seal; &
updated the oil-tube) at about 115K.
I changed it out of FUD (fear, uncertainty, & doubt) & PM (preventative
maintenance), even though when comparing TDC on the crankshaft and that
on the camshaft it was less than 6-degrees off. I believe anything
greater than 6-degrees is the magic number, please someone correct me if
I got my parts through Randy Steele of European Parts Specialists
(www.epsparts.com / 805.683.4020) and his prices were very competitive.
I believe all the above parts came to *under* $250, with about $60 in
sealants alone (two different MB's sealants, silicone-based &
anaerobic). If you just do the timing-chain & tensioner, I bet you can
do it for under $125.
You will need to rent a chain crimper-tool as well (Randy has this).
Definitely get the factory manual, and study it. You will need another
person's assistance to feed the chain so it won't slip (my wife helped),
and remove the spark-plugs to ease turning the crankshaft in the
CLOCKWISE direction (during re-assembly coat only the spark-plug threads
with anti-seize paste & coat spark-plug boots with dielectric grease).
CAVEAT- mark position of cam-shaft sprocket when crankshaft-pulley is at
TDC. This will be your reference point.