Just added uicp... Standstill rev 2500~ 3000 rpm and crusing around same rpm I hear the noise..

 

Seems like maybe it could be the clutch bearing?

 

I have a very similar problem... I only notice a noise when on the highway driving next to a concrete barrier. On the gas there's no metal rattling noise, but when I let off it rattles pretty loud. The weird thing is that it seems to be intermittant. I don't notice any performance issues though i.e., no vibrations. I know after installing my ACT clutch and flywheel I started hearing a low key version of this noise so I just atributed it to the clutch. Now that I'm pushing more power I'm wondering if it could be the drive shaft, carrier assembly, or even just the drive shaft center support bushings? She does have 102K miles on her now...