The car is quiet from cold, then after a period of running a knock or a scrape appears from under the floor, and it may fade again once the car cools. That pattern is the signature of a part that has grown into something it does not touch when cold. Nothing has worked loose. A gap that existed at build temperature has closed.
Establish first whether the sound follows temperature or follows the road. A contact that only appears when hot will reproduce on a stationary car after a long idle. A contact that follows road input appears over bumps regardless of temperature. That distinction removes most of the guesswork before a spanner is picked up.
An exhaust is the longest unrestrained metal run on the car. Fixed at the manifold and hung on rubber at the rear, it is asked to grow along its length while both ends stay roughly where they were built, so the whole run shifts rearward and sideways in small amounts. Clearances that were comfortable on the bench become tight in service.
The movement follows the line of the run. A section passing close to a chassis rail, a tunnel brace or a tank strap closes its gap in the direction the pipe is being pushed. That is why contact usually happens at one specific point rather than along a whole length, and why one side can touch while the other clears.
A heat shield that has lost a fastener buzzes and scrapes in the same area, and often worse when hot because the shield softens. A failed hanger lets the system sit lower and touch cold as well as hot. A slipped slip joint, a loose clamp and a collapsed baffle all produce noises that get mistaken for body contact.
The separating test is cold clearance. If the system has clearance cold and touches hot, the hanger and clamp questions are largely answered and expansion is the mechanism. If it already touches cold, the run is sitting wrong and heat is only making an existing fitment problem worse.
Put the car on ramps cold and work along the run with a strip of stiff card, noting every point where the gap is smaller than the rest. Look for the witness mark: metal that has been rubbing is brighter and cleaner than the soot and dust around it. That mark localises the contact even when the noise is intermittent.
Then run the car until the exhaust is properly hot, let it idle, and look again with a light and a mirror. Do not put hands near a hot system. Compare which gaps have closed. A gap that has gone from comfortable to touching identifies the point, and moving that section is the fix. The exhaust pipe range covers the straight and bent lengths used to re-route a run.
Titanium and stainless do not grow by the same amount for the same rise in temperature, and a thin-wall titanium system carries less mass to hold its shape. A layout that worked in stainless may not carry the same clearance margin once it is made in titanium, so a titanium run deserves its own check rather than an assumption. The titanium exhaust system pages describe the sections most often affected.
The material also changes how the contact behaves. A titanium section that rubs can wear through at the contact point faster than a heavier stainless wall, and the sound is drier and sharper. Neither material should be left in contact, but the thinner wall makes the urgency higher.
The durable answer is to give the run somewhere to go. A slip joint or a flex section near the point where movement accumulates lets the pipe grow without driving a hard section into the body. Hanger position matters as much as hanger strength: a hanger that holds both ends of a hot section rigidly forces the growth into the middle.
When a custom run is laid out, set the clearance with the system hot, or leave enough margin that the cold gap is obviously generous. Fabricators who build the same chassis repeatedly know where the tight spots fall, and a drawing that marks those points is worth more than a general instruction to keep clearance.
