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.