Metal moves as it heats, and a joint that is tight cold can open up as the parts grow at different rates. A flange, a gasket and a clamp do not expand by the same amount, so the assembly finds a new shape at working temperature. That is why a leak can be silent on the driveway and obvious at the end of a drive.
The reverse case confuses people just as often: a joint that ticks when cold and goes quiet when hot is usually a gap that closes as things expand. Both say the same thing: the joint has movement in it, and the answer is to correct the joint rather than add clamp load.
An exhaust tick follows the firing order, so it speeds up with engine speed and stays in step with it. A valvetrain or injector tick does the same, which is why the two get confused. The difference is where it comes from and how it responds to load: a leaking exhaust joint is often loudest cold and under light throttle.
Put your ear near the wheel arch and the floor rather than over the engine. Exhaust noise reflects off the ground and comes back up through the body, so it sounds like it is under your feet. If a noise sits clearly at the top of the engine and does not change when you rev at a standstill, you are listening to the wrong system.
The most useful tool for a leak you cannot see is a gentle air supply, not a powerful one. Feed low-pressure air into the tailpipe with the system cold and run your hand along the joints from front to back while somebody keeps the supply steady. A leak pushes air out against your hand, and the sensation is unmistakable.
Low pressure matters because a strong supply pressurises everything and blows past the joints, which hides small leaks and can push a gasket out of place. A shop vacuum set to blow is usually enough, used cold with the engine off.
A leak that has been there for a while leaves a map. Soot escapes at the gap and stains the surrounding metal in a fan, and the point of the fan is the leak. On a titanium exhaust system the contrast between the stain and the surrounding surface is usually clear enough to photograph and compare later.
Check the downstream side of every joint, not just the face you can see. Many leaks weep along the inside of a flange and appear on the far side, or track under a clamp band and show at its edge. When you find a stain, look for its partner mark on the mating surface, because the pair tells you how much the joint has been moving.
A slip joint leaks when the overlap is too short or the clamp does not seal across its full width. It usually leaks intermittently, at particular combinations of temperature and load, and it often leaves a stain running lengthways along the pipe. A weld leak is fixed in place, so it is consistent and its stain stays local to one point.
That distinction changes the repair. A leaking slip joint can often be solved by cleaning the overlap, seating the pipe deeper and using a clamp that covers the joint properly. A leaking weld has to be cut back to sound metal and closed again, and bridging it with sealant usually moves the leak further along the run.
Set a limit before you start. If the air method and the soot trail have not found it after a careful pass, stop adding sealant and stop tightening. Repeated attempts to close an unidentified leak usually distort the joint that was fine, and then you have two problems and no diagnosis to show for the afternoon.
Take the section out and inspect the faces on a bench, where you can see them straight on and check them against a straight edge. Most leaks that survive an on-car search are a warped flange, a gasket fitted the wrong way round, or a crack at the edge of a weld. All three are obvious once the part is in your hands.
