A pipe that clears a subframe by a comfortable margin on a cold morning can be touching it at the end of a drive. The pipe grows along its length and across its diameter as it heats, and it grows in whichever direction the hangers allow. The gap you measured is a starting condition, not the condition that causes the noise.
This is why clearances that look generous on a lift still produce witness marks. The system does not expand evenly; it moves towards whatever is nearest, because that is the direction with the least resistance once the hangers have taken up their slack. Judge clearance at working temperature, or do not judge it at all.
The front of the system moves most, because that is where the heat is concentrated and where the pipe is longest between supports. The places that catch people are the bellhousing area, the steering shaft, the fuel and brake lines running along the floor, the rear subframe, and the space above the rear axle where the pipe arches over.
The other category is anything that is itself moving. A pipe can be clear of a fixed bracket and still touch a suspension arm under load, or touch the body only when the car is fully loaded. Check clearance with the car sitting on its wheels and, if the usage allows, with weight in the boot and a passenger aboard.
You do not need to measure a hot system. You need to read what it has already done. A polished spot, a bright edge on a bracket, a clean stripe through dirt or a scuff in underseal is a record of contact that is more reliable than any gauge reading taken at the wrong temperature.
Wipe the suspect areas clean before a drive, then look again afterwards. Fresh contact shows as a mark on a clean surface, which removes any doubt about whether a mark is old. This is the most useful thing an owner can do in the first week, and it costs nothing but a rag and a few minutes on the ground.
A shield fitted to solve a clearance problem can create one. Shields are thin, they sit on standoffs, and they vibrate. If a shield ends up closer to the pipe than the original gap was, it becomes the thing that touches, and the noise it makes sounds exactly like the noise you were trying to remove in the first place.
Fit an exhaust heatshield with its own clearance in mind, and check that its fixings do not pull it into contact with the pipe as they are tightened. The same applies to a wrap or an exhaust heatblanket: adding material to a tight space changes the clearance, and it has to be checked hot like everything else.
Most real damage in the first week happens to things that are not exhaust parts. Bumper plastics soften, underbody coating melts and drips, wiring conduit deforms, and a rubber boot near a pipe can go hard and split. None of that appears in a clearance check carried out on metal alone.
Look specifically at the nearest non-metal item to every hot section, and look at it after a drive rather than before. A deformed edge, a glossy patch where the surface has softened, or a drip hanging off a seam is a clearance failure that has already happened, even if the pipe itself has never touched anything.
The rule is that the tightest gap in the system should sit somewhere you can inspect, not somewhere hidden. If the smallest clearance is behind a shield, above a tank or inside a bumper, you will not find the contact until it has worn through something. It is worth rearranging the routing to put the critical gap where you can see it.
When a gap cannot be increased, change what is on the other side of it. Move a line, add a standoff, trim a plastic edge, or change a hanger so the pipe rests in a different position. Clearance is a decision made during the build, not something to be negotiated after the system is bolted up and the noise has already started.
