My 2003 320ci has had the same problem since I bought the car (second-hand
with 12000 miles). Three BMW dealers have had a look at the problem, but
can't resolve it.
On accelerating, in any gear, I can hear a quiet rattle from the front of
the car between 1400 and 2000 RPM. It does it when the car is cold or hot.
One dealership suggested it maybe the ERG (Exhaust Gas Recirculation) unit.
Another suggested it was a valve in the inlet manifold.
It's annoying, more than anything.
The stainless steel exhaust in BMWs is a nice touch, but if they didn't
use fairly thin mild (non-stainless) steel hangers in the vicinity of
the transmission crossmember that rust through and then rattle, leaving
the exhaust system unsupported in that area, they would be nicer yet.
Take a look and see if that's where the problem lies. The part is
inexpensive, but if not replaced stress might crack the (doubtless *not*
...and note especially the part labeled "2".
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