The sound you hear is normal, and to explain why requires some kneebone-connected-to-the-shinbone background. Your NC’s crankshaft turns the clutch, which is attached to the transmission’s main shaft, which turns the transmission’s countershaft on which the countershaft sprocket is mounted, which connects to the rear wheel via the drive chain. When the transmission is in neutral, the main shaft and countershaft aren’t directly connected, and only the main shaft is rotating. When you pull in the clutch, the fiber plates that ft into slots in the crankshaft-driven clutch basket are pushed away from the steel plates slotted into the mainshaft-mounted clutch hub, and the main shaft stops rotating—in theory.
In reality, the oil between the clutch’s fiber and steel plates creates drag, and the clutch hub and main shaft keep rotating. When you shift into first, the gears on the rotating main shaft engage the ones on the motionless countershaft, and the main shaft and clutch hub stop abruptly, sometimes with an audible clunk. The noise is worse on some bikes than others— older Harley’s sounded like a hand grenade went off in the transmission case. Bikes with dry clutches—most BMW boxers, a great number of Ducati’s—don’t exhibit this issue because this type of clutch releases cleanly.
All of the above also applies to shifting from neutral to second while standing still. The common denominator is the motionless countershaft. Once you’re under way both shafts are spinning, and gear shifting is quieter because one shaft doesn’t have to come to a complete halt before engaging with the other.