Strange sound while driving.

[Meskij]

I have a 2001 S with 38k miles on it that makes a sound I can only describe as "an empty tin can rolling on asphalt". It seems to only do it in 2nd or 3rd gear while cruising at speeds around 25-35mph. I live on a military base so this is a good portion of my daily driving. As soon as I let off the gas the sound goes away. It can happen 2 to 3 times in one driving session and then disappear for a week or more. I think I remember seeing a post somewhere else where someone with a similar problem diagnosed it as a valve in the engine compartment that needed replaced, but have no idea where I might have seen it. My last oil change revealed no FOD, metal or otherwise, in the oil and I'll be changing it again this weekend to see if that's changed.

Has anyone ever had this issue and/or know how serious it is, as well as what it takes to fix?

Thanks,

-Joe