Today seems to be a day of thread revival :)

Tom, help us to help you a little. Photos always help immensely trying to understand a machine from across the pond. The term "3040t" describes a generic form factor of typically Chinese machines without any clear indication of the actual machine build.

You mention a noise, a turning knob? (is this attached to the motor?) - I'm going to assume that the motor has two shafts - one driving the drive screw and the other with a rotary knob affixed. And trying to move the X-Axis the knob rotates?, but the bed does not move? If so you might be right with your thought process, but I'd hazard a guess that there's two devices to investigate:

1) Motor coupler - connects the motor shaft to the lead screw. That is normally fixed by two grub screws, 90 degrees opposed on the lead screw; and a second pair of grub screws tightening the coupler to the motor shaft. If these are loose then the motor won't drive the drive screw.

2) Ball nut. Or trapezoidal nut, or delrin nut, all depends on the machine build. But, yes, a nut assembly that fixes to the bed that traverses the drive screw. If that's stripped (possible with a delrin nut) then it could explain your observation.

You're going to have to get your hands dirty and a screwdriver out to look at the mechanical behaviour of the machine.