When posting a message try to give as much info of the vehicle such as what
engine size!!Without hearing this noise it would be hard to say what the
noise is. If it is a purge valve opening and closing it could give you a
knock sound. The noise could even be due to carbon buildup.
Carbon buildup is a relatively new condition that occurs during the
combustion process and results from certain compounds in gasoline. These
compounds form deposits on the top of the piston and cause contact between
the cylinder head and the piston. The condition affects all gasoline engines
and is industry wide.

The brand of fuel and the type of detergents used by the gasoline supplier
significantly contribute to the condition. Also, vehicles used in city
traffic or light duty cycle operation are more prone to incurring the
condition.

Glenn Beasley

Chrysler Tech

My kid's '94 Caravan ES has a faint but noticeable knock at idle,
really sounds like the bottom end.

 The oil had been changed recently and I wanted to know what brand was
used but he didn't know.

 I put some STP in it just to see if it would change the sound, and it
did -- but he said that after while it came back.

 Has anyone worked on these enough to have noted if it's common for
them to sound like that?  It's faint enough that many people would
probably not even notice it -- but definitely there.  Goes away as soon
as RPMs are increased.

 This one has about 150 K miles and has always been serviced regularly
since new at large local dealership, until about a year ago when it
began getting service elsewhere.