Assuming you don't have any obviously loose parts, you might be hearing be what is called "gear rollover" in the transmission. Google search "gear rollover" The nv5600 is a medium duty truck transmission, full of alot of big gears and shafts. The right combination of load,gear selection,rpm and engine harmonics will make some of the gears without a load on them rattle/vibrate on the shafts. Sometimes an oil change will help quiet the noise.
What Sam said.
Gear Rattle is a repetitive metallic impact or rapping noise which occurs on a manual transmission powertrain
when the vehicle is lugging in gear. The rattle noise intensity generally increases with transmission operating
temperature and engine torque, and decreases with increasing vehicle speed. Since the gear ratios have been
designed to achieve maximum fuel economy, there may be instances when gear rattle is distinctly noticeable
under lugging conditions. This, however, is not detrimental to the engine or transmission provided that the gear
selected is appropriate for the vehicle speed.
Replacement of transmission components will not correct neutral rollover rattle.
Neutral Rollover Rattle has the same characteristics as gear rattle except that rollover occurs with the engine
idling, transmission in NEUTRAL, and the clutch engaged (clutch pedal released). The rollover noise intensity
increases with transmission operating temperature and engine torque load resulting from air conditioning and
generator. Neutral rollover noise is not detrimental to the engine or transmission. In vehicles where the engine
idle speed is below specification or rough, a harsh clattering noise like loose marbles in the transmission may
Rollover Noise caused by engine torsional vibrations and Clutch Release Bearing Noise are sometimes
mistaken for Transmission Bearing Noise. Neutral rollover noise will disappear when the transmission is
engaged in gear. Due to a constant running clutch release bearing (used for the hydraulic clutch release
mechanism), noise caused by a worn or damaged clutch release bearing will be noticeable with the clutch
engaged or disengaged (clutch pedal released or depressed). Transmission service will not eliminate gear
rollover noise or clutch release bearing noise.
While verifying the condition, determine whether the noise is gear rollover noise, release bearing or some
other transmission related noise.
Gear rollover noise, inherent in manual transmissions, is caused by the constant mesh gears turning at engine
idle speed, while the clutch is engaged and the transmission is in NEUTRAL; and release bearing rub are
sometimes mistaken for mainshaft bearing noise.
Gear rollover noise will disappear when the clutch is disengaged or when the transmission is engaged in gear.
Release bearing rub will disengage when the clutch is engaged in the event that a bearing is damaged, the
noise is more pronounced while engaged in gear under load or coast than in neutral.