A rhythmic tick or clunk, once every second or two, coming from the extruder motor rather than from the nozzle. Usually paired with visible gaps in the extrusion at the same rhythm.
What the noise is
A stepper motor holds position by magnetic force. Ask it for more torque than that force can deliver and the rotor slips to the next pole — and the mechanical snap of that slip is the click. Each one is a small amount of commanded filament that never entered the hotend.
So the sound is not a warning that something might go wrong. It is the report of plastic that is already missing from the print.
The related sound at the other end of the machine is quite different: fine pops and hisses from the nozzle itself are steam, not torque, and belong on popping and hissing.
When it starts is the diagnosis
On the first layer, immediately. The nozzle is too close to the plate and there is physically no room for the commanded volume. Raise the Z offset — this is by far the most common version, and it is not a hotend problem at all. See first layer too squished.
As soon as the print speeds up after the first layers. The profile is demanding more melt per second than the hotend can produce. Slow down or raise the temperature; the arithmetic is on under-extrusion.
Twenty minutes to an hour into every print, at no particular height. Filament is softening above the melt zone and jamming. That is heat creep, and it has a distinctive clock.
At random, on a machine that was fine. A partial blockage, a tangled spool, or filament that has been ground flat and no longer grips.
Four branches, one observation. Note which one applies before opening any settings.
Check the cheap things in this order
- Is the spool turning freely? Lift the filament by hand above the extruder and see whether it moves without effort.
- Is there a flat on the filament? Pull a length out and look. If the gear has already cut into it, see grinding — and cut that section off before retesting.
- Does filament push through by hand at temperature with the idler released? If yes, the hotend is clear and the problem is upstream or a flow demand issue. If no, the hotend is blocked.
- Is the hotend actually at temperature? A failing heater cartridge or a thermistor that reads high produces a melt that is cooler than the display claims, and the extruder is then pushing part-solid plastic. If the machine also halts intermittently, read thermal runaway rather than treating this as a flow fault.
Verify the temperature independently if you suspect it
A thermistor that has drifted or partly detached from the block reports the wrong number confidently, and every symptom then looks like a clog. An infrared thermometer aimed at the block, or the simple test of extruding at a temperature you know a material flows at, will separate "the plastic is not melting" from "the plastic cannot get out".
This is worth doing before dismantling a hotend, because a cold hotend and a blocked hotend feel identical from the extruder's side.
Do not tighten the idler
The reflex when an extruder is not delivering is to clamp down harder on the filament. It converts skipping into grinding, which is a worse and messier failure, and it fixes nothing because torque was never the missing ingredient — resistance was the problem.
Set tension to specification and go and find what the extruder is pushing against.
When the motor itself is the cause
Rarely, but it happens: a stepper driver set to a low current, a driver overheating and reducing output, or a connector on the extruder motor that has worked loose. The tell is that the clicking is unrelated to what the print is doing — it continues when the machine is idle and being purged, at low flow, with the filament removed.
That version is genuinely a machine fault rather than a flow fault, and stepper motors running hot covers the thermal half of it.
Materials that provoke it
Fibre and particle-filled grades, which raise melt viscosity and block small nozzles. Any material printed near the bottom of its temperature range, where viscosity is highest. Flexible filaments, which buckle rather than push and whose clicking usually means the path needs constraining rather than the settings changing. And PLA in an enclosed machine, where the clicking is nearly always the first audible symptom of heat creep rather than anything happening at the nozzle.