A tick that speeds up with the engine is generated once per cycle, and its rate tells you how many times per revolution. That is why the first thing to establish is whether it matches engine speed exactly or a multiple of it. One tick per revolution points at something rotating; a faster flutter points at something pulsing per cylinder.
Exhaust ticks are in the second group. They are gas escaping a small opening that is opened and closed by the pressure pulse, so each pulse produces a small chirp and the series is heard as a tick. That is why it rises with revs and can vanish at high rpm, when the pulses merge into a hiss.
A pinhole in a weld is the classic cause, most often at the root of a joint where penetration was incomplete or shielding was lost. The hole can be tiny and only leaks when the pipe is hot and pressurised by the pulse. Cold, the metal contracts and closes it, which is why the tick worsens once warm.
Finding one takes patience. With the engine idling, hold a hand close to the suspect joint without touching it and feel for a pulse of hot gas. Then look for soot: a pinhole leaves a small dark speck with a tail pointing where gas escapes. Good TIG stainless steel welding with full penetration does not do this.
Not every tick that follows rpm comes from the exhaust. Lifters, injectors and the high-pressure fuel pump all tick at engine speed or a multiple of it, and all sit in the engine bay. The separating test is location: an exhaust tick is loudest underneath or at the tailpipe.
A second test is the tailpipe itself. A leak upstream of the muffler is still audible there but fainter, because the muffler attenuates it. A top-end tick will not be audible at the tailpipe at all. That comparison usually decides which system you are dealing with, for the cost of a walk to the back of the car.
A crack in a manifold or downpipe behaves like a larger pinhole, with a harder tick and a slight rasp that worsens under load. Cracks start at a weld toe, a heat-affected zone or where a bracket was welded on, and they open as the part heats and the two sides move apart.
Look for a line of discolouration along the joint; a leaking crack leaves a dark carbon trail. Where the crack sits at a bracket, the bracket is often the cause, because a rigid bracket on an expanding pipe concentrates stress at its weld. Reinforcing the area without changing the restraint just moves the crack along.
A failed flange gasket produces an intermittent tick, worst at idle, because manifold pulses are strongest at low flow. The sound is a soft puff rather than a sharp tick, and it usually comes with a faint exhaust smell in the cabin. Blown manifold gaskets are common on cars that have had exhaust work.
Look at the flange face for a black streak and feel for a pulse at the joint with the engine idling. A flat flange and a correct gasket will not leak; a flange distorted by welding will leak however tight the bolts are. That is why faces are checked for flatness after welding rather than assumed.
Work from the engine outwards: manifold, each joint, then the tailpipe. The point where the tick is loudest is the leak, and the point where it first becomes audible as you move forward is the boundary. Loud at the tailpipe but not at the manifold means the leak is behind that joint.
If you cannot localise it, a length of hose used as a stethoscope excludes surrounding noise better than a torch does. Move the end slowly along each weld and listen for the change. Once located, the question is repair or replace: a stainless steel exhaust muffler with a leaking seam is a replacement, because the seam is not reachable from outside.
