Buzzing that starts immediately after a cat-back is fitted is rarely a defective part. It is a new geometry finding its clearances. The factory system hung in one place with particular gaps, and the replacement has different bends, a different case and different hanger positions. Something in that arrangement is now close enough to touch under vibration.
Separate a buzz from a drone first. A buzz is fast, thin and often located to one side; a drone is a low pressure in the ears. A buzz that arrived with a new part is almost always contact or a resonant panel, and it changes when you press the body above it. Drone will not.
The usual suspects are the top of the case against the floor pan, the over-axle pipe against a suspension arm or tank shield, and the tip against the bumper cut-out. All three are places where the original had clearance. The contact need not be heavy; a pipe resting lightly on a shield buzzes at the panel's frequency, not the engine's.
Get the car on ramps with the suspension loaded and look along the run with a torch. Bright rub marks, polished spots and wiped-clean dust are the evidence. Watch the highest point of each bend. An exhaust heatshield bent slightly during fitting is a common new buzz, because it is thin and sits where the pipe passes.
A clamp tight enough to hold but not seated correctly will buzz. So will a slip joint pushed fully home and left metal to metal with no allowance for movement. Clamp noise is usually a sharp repeating tick; a buzzing panel is continuous. If the buzz stops at idle and returns as soon as the car moves, suspect a joint moving under load.
Sleeve clamps are frequent offenders because they are easy to over-tighten and they distort the pipe. A distorted pipe leaves a step inside, and a step is both a restriction and a noise source. Look at the clamp band for a witness mark that is not straight, which shows it was pulled off square.
Structure-borne noise travels through the body, so a contact at the rear can be heard as a buzz from the front footwell. Owners often say the noise is under the dashboard when the contact is at the rear axle. The panel you hear is the one resonating, not the one being struck.
Use your hand as a damper. With the car idling, press firmly on panels one at a time: boot floor, rear seat base, parcel shelf, the floor above the rear section. Where the buzz changes or stops, you have found the panel broadcasting it, and the contact is within reach of that panel.
A kit hung from the original hanger points sits differently if the new section is heavier or longer. That changes the angle of every hanger and can pull the system into contact where the bench showed clearance. The tell is a buzz that is worse on some road surfaces than others.
Check preload by pushing up on the pipe at each mount; it should move a little and return, not sit under tension in one direction. If one hanger is clearly pulling, the system is fighting itself. Where the run must be altered, an exhaust pipe section made to the clearance actually available beats spacing the old one down with washers.
Start with clearance, not the part. Loosen the clamps at the joints, let the system settle under its own weight, then retighten from the rear forward. That one step resolves a surprising share of post-fitting buzzes, because it lets the pipe find its natural position instead of staying where it was first pushed.
If the buzz survives, look at panels rather than the pipe. A loose shield, a cracked bumper bracket or a boot floor missing a fixing will buzz with any exhaust, and would have buzzed with the old one if the old one had been loud enough to excite it. Buzzing after a cat-back is usually clearance, sometimes a clamp, rarely the part.
