An exhaust that sounds different after a routine service has usually had something disturbed rather than something fail. A service involves lifting the car, moving it on a ramp and often removing a heat shield or brace to reach a filter or drain plug. Any of those can leave a joint loose or a shield one bolt short.
Establish the character of the change first. Louder and harsher points at a leak. Softer and duller points at a restriction or a collapsed baffle. A new rasp or buzz points at a contact or a loose component. The direction of the change is the fastest way to sort the fault.
A flange or slip joint taken apart and reassembled can leak if the gasket was reused, the faces were not cleaned, or the fasteners were tightened in the wrong order. The result is a harder, thinner note with a hiss at the edge, usually worse under load. It is the commonest cause of a tone change after a service.
Look for soot at the joint, a dark streak radiating from a flange face or sleeve. Then listen with the engine idling, holding a hand near the joint to feel for a pulse. A leaking joint shows both. If the note changed but there is no soot and no pulse, the cause is elsewhere.
If the exhaust got quieter and duller, a baffle or packing has probably shifted. That is a coincidence of timing rather than a result of the service. Packing migrates and compacts with use, and a baffle can break its welds and drop into a position that blocks part of the flow.
The tell is that the note is not just quieter but less defined, with the higher frequencies gone. A stainless steel exhaust muffler that has lost its packing sounds hollow rather than crisp. No external check proves it, but a rattle from inside the case when the system is rocked by hand is a strong indication.
A shield refitted without all its fasteners, or a brace put back with the wrong bolts, produces a buzz or rattle that was not there before. The sound is usually at a specific rpm and owners describe it as a vibration rather than a noise. It appears after a service because the service is when the part was last touched.
Check every fastener that was removed. A missing bolt leaves a shield free to flap; a bolt present but not seated leaves a gap that opens and closes with vibration. Look for clean metal around bolt holes, which shows where a fastener has been moving, and check that no washer has been left behind under a brace.
A sensor removed and refitted can be cross-threaded or left loose, which leaks at the boss rather than changing the exhaust note. The sound is a tick rather than a tone change and it is localised to the sensor. Worth checking because it is easy to create and easy to miss.
A subtler cause is a pipe dented during the service. A ramp arm placed under a pipe flattens a section, and a flattening changes both flow and sound. Look along the underside for a fresh scrape or a flat spot. Replacing a crushed section with a proper stainless steel exhaust pipe restores the note and the flow.
Before doing anything, confirm the change is real. A car that has just been serviced feels different because you are listening for it. Drive the same route at the same speed and compare against a recording made beforehand if you have one. A difference only at idle and only when cold may be seasonal.
If the change is real, work from the joints outwards: every fastener that was touched, soot, a pulse at each joint, and fresh damage underneath. Most answers are a joint needing a gasket or a shield needing its last bolt. A tone change after a service is usually a small assembly fault, and worth fixing before a leak erodes a flange face.
