if you haven't had the tensioner updated the cause of the rattle is three-fold:
1) the chain has stretched and needs to be replaced
2) the tensioner is the old style without an internal ratchet which maintains tension until oil pressure is built up.
3) in the absence of correct tension on the timing chain, the chain actually hits the valve cover until tension is re-achieved.
this is a common problem with earlier W201s.
you will need to replace the chain and tensioner (obviously) and quite possibly the timing chain rails and guides. it depends on the amount of wear that has occurred on these components. ensure that you get the upgraded tensioner.
go to www.190revolution.net
and search there. LOTS of info on that site.
this noise is not associated with the drivebelt tensioner.
the rattle is t