knocking noise in 1st gear

[dozu]

'90 240 sx 5sp, recently developed a slight knock noise when the car accelerates in 1st gear, only happens around 1500-2000 rpm. only happens when the car is warmed up.

Clutch is new, plenty of grease in differential, shifting is smooth, all gears running smoothly, no signs of any operating trouble.

What could be the cause of this noise?

[dozu]

by the way, it sounds like the noise comes under the shifting knob.