Hot pipes tick
Thermal expansion against friction, and why the noise arrives in bursts.
Hot pipes tick because the metal is trying to grow longer as it warms, and wherever it passes through a bracket or clip that grips it tightly, it cannot simply slide through smoothly, it sticks, builds up force, and then suddenly slips a tiny distance, and each of those small sudden slips is heard as one tick.
Growth held back by a gripping bracket
A metal pipe carrying hot water undergoes thermal expansion as it heats, growing slightly longer than it was when cold, and that growth has to go somewhere along the pipe's length. Where the pipe passes through a loose-fitting hole it can usually slide without much trouble, but wherever it is held firmly, by a clip, a bracket, or simply squeezed against another fixed surface, friction resists the sliding. As the pipe keeps warming, the section held by the bracket stays put at first and simply builds up internal stress, held in place by the same grip that resists any small motion. Once the force trying to push the pipe through exceeds what the grip can hold, the pipe suddenly slips forward a small amount, releasing the stress in an instant, and that sudden release is the sound. The pipe then sits still again, gripped once more, building up stress from the next bit of expansion until it slips again, which is why the ticking arrives as a series of distinct clicks spread over minutes.
The amounts involved are small but far from negligible. Copper grows by about seventeen millionths of its length for every degree it warms, so a ten metre run of heating pipe going from room temperature to around seventy degrees grows by nearly a centimetre. Most of that has to pass through one or two tight points along the route, and pushing even a millimetre of it through a gripping clip is enough to store up a sharp, clearly audible tick.
A trainer squeaking on a gym floor
A rubber-soled shoe pivoting slowly on a dry gym floor rarely slides in one smooth motion. It grips the floor, resists the twisting foot above it, and then suddenly gives way with a short squeak before gripping again a moment later, producing a series of distinct chirps even though the foot is turning at a steady rate. The floor stays just as grippy throughout and the foot never pauses. Friction holds the rubber still until the twisting force built up in it becomes too great to resist, at which point it slips a small distance almost instantly and the grip resets. A hot pipe does the same thing at a bracket, with thermal expansion supplying the force in place of a twisting foot, and the release heard as a tick instead of a squeak. In both cases something held too firmly to move smoothly moves instead in small sudden jumps, each loud enough to notice, however quiet the steady process driving it.
Ticking on the way down, and how plumbers quieten it
The same process runs in reverse once the water stops flowing and the pipe begins to cool, shrinking back toward its original length and sticking against the same bracket in the other direction until the shrinking force snaps it back a small step. That is why a heating system often ticks a second time, quietly, long after it has been switched off.
The expansion itself cannot be prevented, so the fix is to stop the pipe being gripped tightly enough to stick and slip. Plumbers commonly loosen or pad the clips that hold heating pipes, sometimes lining them with a soft plastic sleeve, so that the pipe slides through as it grows without ever building up enough stress to release suddenly. A pipe run straight through a tightly fitted hole in a wooden joist ticks at the joist, and the remedy there is the same: widen the hole slightly or wrap the pipe so it can move freely. The pipe still grows by exactly the same amount, since that is set by the metal and the temperature, and the only thing that changes is whether the growth happens as one smooth, unnoticed slide or as a string of audible releases.
A run with several tight points can tick from more than one place in turn, each bracket releasing at a slightly different moment as the stress along the pipe reaches its own threshold, which is why the ticking during a heating system's first few minutes often seems to wander from one part of a wall or floor to another.