[Hammered 77]

The carpet will definitely dampen the noise. The transmission housing resonates any noise it gets whether it's from the bearings, the gears or something else. Depending on how things heat up, I suppose the bearings could tighten up a little. If it's really only in 5th gear, I'd also look at the gear set. Again as it heats up the gear mesh will become tighter, and that will definitely affect noise.