A rattle that is there for the first minute and gone once the engine is warm is a clearance that closes itself. Metal grows as it heats, so parts that touch when cold can sit apart at working temperature. The noise is not a failure warning; it is a gap that exists in one thermal state.
Establish where the noise lives. A rattle from the engine bay follows the engine, so it stays put over bumps and changes with revs in neutral. A rattle from under the floor moves with the body and often quietens when you press a hand on the panel above it.
A cold exhaust is shorter than a hot one and hangs slightly differently. The usual cold-only rattle is a pipe or muffler case resting against a heat shield, chassis rail or bumper bracket until the system grows into free space. Look for a polished witness mark or a bright scuff.
Separate that from a loose baffle, which rattles at a different rhythm and ignores temperature. A baffle sounds hollow and dull and often appears under load; external contact sounds thin and metallic at idle. The stainless steel exhaust mufflers on a good system are rigid enough that the case is rarely the culprit; it is what the case touches.
Rubber hangers are stiff cold and soften as they warm. A hanger already carrying preload, with the system pulled sideways or down to reach it, transmits that load into the body as a rattle until the rubber relaxes. The noise comes from the mount area and fades in a patterned way.
Preload also explains a noise that appears after a part is replaced rather than after it wears out. With the system cold, push up on each hanger in turn. One doing real work feels tight and moves the whole pipe; a correctly loaded one flexes and lets the pipe settle without dragging the run with it.
Heat shields are thin, fastened at a few points, and the commonest source of a cold-start rattle on an otherwise fine car. A shield that has lost a fixing flaps against the floor or the pipe, then stops as it heats and expands against its remaining fixings.
A loose bracket knocks rather than buzzes and answers to torque rather than temperature. Rev gently in neutral from cold and listen for where the noise starts. If it begins at a particular rpm and stays while that rpm is held, suspect a bracket. If it is there at idle from the first second, suspect contact that closes with heat.
Slip joints and flange faces are the cold-only source most often missed. A slip joint relies on interference and clamp load, both of which change with temperature. Cold, the joint is marginally loose and ticks as gas pulses pass; hot, expansion takes up the clearance and the noise stops.
Look for soot tracking, a thin dark streak radiating from a flange face or sleeve. Soot only forms where gas escapes, so a clean joint is not leaking and a blackened one is. That separates a joint problem from a contact problem for the price of a torch.
Do the work before the engine has run. Slide under with a torch and look for witness marks: bright metal on a shield, a polished line on a pipe, a shiny spot on a rail. Then push and pull the system at each hanger. Anything that clicks by hand will click under engine movement.
Only then start the engine and note whether the noise is present at idle or arrives with revs, because that splits the diagnosis in half. Where a section must be replaced, geometry matters more than material: an exhaust pipe that follows the original routing and leaves the same gaps will not reintroduce the fault.
