<E36>, help!! rattling noise under car

[Kwok Seto <kseto@xxxxxxxxxxx>]

Over the past few months, there has been a VERY VERY annoying
rattling noise underneath the vehicle.  The noise appears whenever
I go over small bumps in the road and sounds like a metal joint is
loose.  The rattling seems to appear once I drive for about 5-10 min
and then you can hear it clearly.  I took this problem to a local
mechanic today and we sat there for 2+ hours trying to locate the 
noise, but nothing came about.  If anyone has any ideas at all, please
email me your suggestion at kseto@domain.elided

Thanks in advance,

Kwok Seto

1993 BMW 318is 

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