My 2004 GC with 3.8 liter 127,000 miles sounds like a marble rolling around in a tin can. The sound comes and goes, but loudest at low RPM. The dealer is lost, and I had them take the belt off and run it, and the sound was gone. They could not find any problems with any of the pulleys, so should I be worried? Any suggestions?

 

You should be worried, I suggest you find another mechanic that is more competent. Since it's been diagnosed to be something off the belt, replace the water pump, alternator, power steering pump, A/C compressor, idler and tensioner pulleys and see if the noise goes away.

 

Get yourself a stethoscope or make one out of a piece of hose with a bolt shoved in one end. Touch the bolt to each accessory and put the end of the hose in your ear. It should amplify/isolate the noise.

 

Not that this will be of any help but I thought I'd relate this true firsthand story.

Back in the late 70's I visited the Southampton LI Dodge dealer (now gone) to order some parts. As I approached the building I heard an engine off in the far end of the yard running wide open. Being a pest by nature I had to inquire as to what that was about. "That car has been a nightmare. Owner came in with an engine related noise complaint and we tried every thing we could think of and could never find the cause" SO? I asked "So we decided to put a brick on the gas pedal to blow it up and warranty it"

And as if that wasn't bad enough....about a week later when my parts came in I asked whatever became of that warranty claim. "That bastard never popped and we got tired of listening to it so we tossed a handful of old bolts down the carb. YOU SHOULDA HEARD THAT SOUND!"

And you wonder why Mopar was closing plants faster than the speed of sound back then. But it does speak well of the durability of their engines to tolerate running wide open for as long as it did. I wonder how much they charged the customer for "diagnosis" LOL!

 

When do you hear the sound? If its at idle, maybe check the above mentioned parts, but if its when you are driving, you could have bigger issues.
Thats what detonation sounds like.
If so, try changing your plugs and/or your gasoline. Detonation ruins engines.

 

My mother in laws Caravan 00 has a similar sound and I found it to be the tensioner. Belt and tensioner had 130k on it.

 

You sure the sound wasn't actually your mother in law? LOL