Vibration felt only at a steady cruise is one that needs a sustained condition to build. At cruise the engine speed is constant, the exhaust is at a stable temperature and the body sits at a fixed height. Any resonance in that state persists, which is why the symptom is repeatable at one speed and absent elsewhere.
The first split is seat and floor against steering wheel. Floor and seat vibration comes from behind the front axle: exhaust, propshaft, rear differential or rear wheels. Steering wheel vibration comes from the front. Getting that wrong sends you to the wrong end of the car.
Wheel imbalance rises and falls with road speed and is usually strongest across a broad band rather than at one point. Exhaust resonance is narrower and often indifferent to road surface. Change gear at the same road speed: if the vibration stays identical, road speed is involved; if it changes, engine speed is.
Exhaust vibration at cruise usually comes from contact that closes only when everything is hot and settled, which is why it can be absent on a short drive and present on a long one. The system grows and the hangers soften. A stainless steel exhaust system with well-placed hangers resists this, but any system can be installed too tight.
A propshaft vibration is a fast buzz through the floor and often the centre console, worst at a particular speed band. Causes include a worn centre bearing, a damaged universal joint or a lost balance weight. Unlike exhaust resonance it often has a cyclic quality, a beat that comes and goes.
Inspect the shaft for missing balance weights, a torn rubber coupling or play in the joints. Try to rotate it and lift it; any perceptible movement at a joint is a problem. If the vibration appeared after clutch or gearbox work, suspect the shaft was disturbed rather than the exhaust, because the two jobs often happen together.
Hangers are the isolation between exhaust and body, and their stiffness decides how much vibration gets through. Too stiff and engine and exhaust vibration reach the floor directly as a fine high-frequency buzz. Too soft and the system moves enough to knock. Neither extreme is a fault in the part.
Owners often fit polyurethane for durability and then find the cabin noisier at cruise. That is the trade-off working as designed. If the vibration started with new hangers, the usual answer is a softer rubber at the rear and a firmer one at the front, locating the system at one end and letting it move at the other.
Check whether the vibration follows a stretch of road rather than a speed. A system sitting low is affected by airflow and by how the underside is loaded at speed. More practically, a system very close to the floor transmits noise and vibration into the cabin far more readily, because the air gap is the insulation.
Measure the gap at the tightest point and again when hot. Where a section must be replaced, routing matters: a stainless steel exhaust pipe bent to follow the floor pan holds clearance better than a straighter universal section forced into place. Clearance is a design output, not something found by adjusting clamps afterwards.
Find a quiet stretch and hold the offending speed. Then do three things in sequence: change gear without changing speed, press the clutch and let the engine drop to idle, and coast in neutral. If the vibration goes when the engine idles, it is engine or exhaust driven.
If it stays while coasting in neutral, it is wheels, tyres or driveline. That sequence costs nothing and splits the diagnosis into halves needing different tools. Only then put the car on a lift and look for witness marks. Most cruising-speed floor vibrations that turn out to be exhaust are contact or hangers, both visible with a torch.
