2002 SC2 noises

2002 SC2 manual transmission

When I start the car after it sits for a while, I get this brief
clunking sound as I start to drive either forwards or backwards (most
noticeable in reverse or whenever I don't give it much gas) . After
maybe 30 seconds to a minute of driving it goes away.

Any ideas? Thanks.

 

Does the sound happen then stop or does it continue for 30 seconds?
If it just happens for a moment, it could just be the sound of the ABS
cycling - it does it every time you start the car. If it is
continuous, it is likely somthing else.

 

No ABS. It continues and changes speed with the motor. It is not a
shuddering, lugging sound like trying to start out in the wrong gear,
just most noticeable when in gear and rolling at or near idle RPM. It
is also not a rock in the tire or something like that - most definitely
a mechanical clunk. The motor also always has some of what seems like
valve train noise, but that is there even when the car is warm. I have
heard that an exhaust leak can sound a bit like "lifter noise" and it
makes sense that it would get louder under load, but not that the deep
clunk would go away quickly. I suppose thermal expansion could seal it
somewhat... Oh well, I am bringing it in and I guess we'll find out.
Cross your fingers.

 

Definitely changes with *engine* speed and not *vehicle* speed?

An easy way to tell the difference is to hold speed constant while
downshifting to a lower gear (even with an automatic transmission) to see if
the noise changes. If it does, then it's related to engine rpm; if it doesn't,
then it's related to vehicle speed.

And if the noise is present only when the car is moving, and varies with speed
(but not engine rpm), it might be the speedometer cable.

 

Dealer Diagnosis: Piston Slap

Recommendation: Do nothing, it is normal for the car. Idle car for a
while until it lessens, then drive at low RPM's until warmup.

 

A thought occured to me. I wonder if the "rattle" caused by this would
affect the knock sensor, causing the timing to be retarded under
acceleration, and be the reason the car seems to have a bit less pep
than my '96? Where is the knock sensor(s) mounted?