'94 Caravan, faint knock

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.
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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
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Boy is my face red... The engine size is a 3.3 -- I always include that bit of info (naturally) -- but I edited my message before posting and inadvertently cut that part out without realizing it and then re-typed the line without it... I know that's a pretty glaring omission, making it much more difficult, if not impossible, to address any engine problem when you don't even know what engine it is. :-)) Thanks for the replies.
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