Radiation.
in some cases if the exhaust is rubbing on the gas tank most new vehicles have poly plastic tanks and most of them have heat shields covering the exhast older vehicles have steel gas tanks
It shields the rest of the car from the heat of the exhaust or radiator .
The cast of Victory Vehicles - 1943 includes: Fred Shields as Narrator
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Samson exhaust systems for motorcycles typically sell from $300 to $800, depending on the model. Samson exhaust systems typically come with full coverage heat shields.
On each side of the engine, four of the bolts that attach the exhaust manifolds to the head, first go through the heat shield, through the exhaust manifold and into the head. These four bolts must be removed to get the spark plug heat shields off.
The purpose of the shield was twofold. First it was a protective device, preventing javelins, arrows or sword thrusts from hitting the man. Secondly, it could be used as an offensive weapon by pushing the opponent off balance.
On a Ford Aerostar there will be heat shields around the catalytic converter and various locations of the exhaust system On the exhaust pipe they are held in place by a rather small width metal band which I just replace with a worm and gear hose clamp once the bands break so I don't have to listen to that " tinny " rattle
Subarus are well known for having rattles in the exhaust shields under the car. There are extensive heat shields on the exhaust pipes that are held on with clamps. Over time these clamps loosen up and their bolts rust out. When the engine runs through certain RPM bands (as in shifting), these shields come into resonance and vibrate. If you get under the car and examine all the heat shields you will probably find the loose one. Replace the missing or rusted out bolts and the rattles will go away.
A heat shield is used to keep high exhaust system operating temperatures from radiating to the floor of the passenger compartment or other vehicle parts that should not be exposed to the high heat. If it is mounted above the catalytic converter it becomes the catalytic converter heat shield. There can be other heat shields through-out the exhaust system not just the converter. The converter or converters, (some vehicles are equipped with more than one) are the hottest part of the exhaust system. Generally heat shields are made of stamped aluminized steel. They are light weight and effective in shielding various parts or areas of the vehicle from the high exhaust operating temperatures. They are also prone to corrosion after a long period of service, they can rattle and make noise once they become rusted from their mounting areas. They should under no circumstances be discarded once deemed broken. Instead heat shields should be replaced as they are needed for above mentioned reasons.
It may be the heat shields on the exhaust system vibrating.
Heat shields are used on cars to protect components and bodywork from heat damage coming from the engine and exhaust system.