Hey, I've got an Oldsmobile Delta 88 Royal, 1988, 3800 VIN C engine,
125,000 miles.  I had all of the fluids changed, transmission and
radiator powerflushed, etc about 1000 miles ago.  Before all of the
fluid flushes, I also Seafoamed the engine, which made it run a LOT
smoother, but I don't think it contributes too much to this problem
(or maybe it does, who knows?).

After a semi-rough start today (where you turn the key and it barely
catches, putters, and complains until it grabs a good idle 5 seconds
later), I noticed the engine makes quite a loud rattle noise at idle
rpm.  Right after I start it, sitting at lights (in neutral or drive,
doesn't matter), or after I park, I hear the noise.  If I step on the
gas pedal AT ALL, the noise goes completely away.  It also takes a
second or two for it to come back after I let off the gas (but it does
come back if I let of the gas and let it coast).  It's loud enough to
get other drivers to notice it when I'm driving, but it doesn't seem
to affect the performance of the vehicle at all.

I tried the "screwdriver-as-a-stethoscope" test, and I didn't really
hear anything from the alternator, but I do hear it loudly from either
the area right below the power steering fluid reservoir (is the water
pump?), and on the rear valve casing, louder on the passenger side
(towards the alternator/etc).  I know my car has camshaft sensor
problems (as in, there's no magnet left...:(  ), but that doesn't
really cause any problems, besides the minor inconvenience of having
to let it idle a bit after cold starts (or else it suffers from gas
pedal "lag" for the first couple of minutes).  This noise I'm hearing
is completely new, and I'm hoping it's something that won't cost an
arm and a leg, like the harmonic balancer pulley.  I can't really
afford to have them pull the engine and replace the timing chain
(though, that'd be a good time to get a new camshaft position magnet,
whatev), but if that's the problem, I'd like to know.

Anyone have any thoughts?