11 Sources of Engine Rattling and How to Fix Them

A mechanic looks at a trouble engine.
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The modern internal combustion engine is a complex ballet of hundreds of parts, all working together to convert fuel energy into motion. When you look under the hood of your car, not much is visible, much less heard—that is, when everything is in working order. An engine rattle is usually a sign that something has gone wrong. Once you identify the source of the rattle, you can take steps to make the proper repair.

1. Broken Belt Tensioner or Chain Tensioner

Drive belts, timing belts, and timing chains need to have a certain degree of tension to work properly. The belt tensioner is typically spring-loaded with some sort of hydraulic or elastomer damper. If the spring breaks or the damper fails, the tensioner may bounce, causing an engine rattle. In this case, you should replace the broken tensioner as soon as possible.

2. Cracked Catalytic Converter

The catalytic converter is an exhaust emissions control device. Inside, a steel or ceramic matrix coated with inert rare-earth metals converts harmful emissions so that they are less toxic. In the case of ceramic-based catalysts, a thermal shock or an impact can cause the matrix to crack. If a piece breaks off, you may hear a rattling in the exhaust. A cracked catalytic converter shouldn’t cause any collateral damage, and replacement is simple, though expensive.

3. Collapsed Valve Lifter

The camshaft drives the intake and exhaust valves. Mechanical valve lifters may be adjusted with shims or an adjusting screw. Hydraulic lifters use oil pressure to maintain proper clearance. If the lifter squishes or can’t hold pressure, the clearance will be too great, causing a rattle. On its own, a collapsed lifter may not cause any damage, though it could cause a cylinder misfire. Replacing the lifter and adjusting the valve clearance will make the rattle go away.

4. Cracked Flex Plate

On automatic transmission vehicles, the flex plate connects the engine to the transmission. At the center of the plate, bolts connect to the crankshaft. Near the edge of the plate, bolts connect to the torque converter. Cracks can sometimes appear around the bolts at the crankshaft. Flex plate diagnosis and replacement requires transmission removal, which can be expensive.

5. Low Oil Pressure

Oil pressure components include variable valve timing (VVT) drivers and hydraulic valve lifters. When there is insufficient oil pressure, these parts may not function, causing rattling in the valves or VVT drivers. Check the oil level first and top it off if needed. If the oil is low, the leak or burning problem needs to be fixed before bearings are ruined or emissions become critical. Otherwise, you may have another problem in the oil pump system.

6. Rusted-Out Heat Shield

Older passenger vehicles may experience age-related issues such as debilitating corrosion. In some places, such as on the catalytic converter or the muffler, heat shields provide protection from exhaust heat. On and around the exhaust system, heat accelerates corrosion. A rusted-out heat shield could fall off, producing a sound like an engine rattle. In that case, the heat shield needs to be replaced.

7. Engine Ping

Engine ping or pre-ignition is caused by hotspots in the cylinder igniting the air-fuel mixture before the spark plug. The two flames collide, boosting the pressure. This is usually caused by low-octane fuel in a high-compression engine, but it can also be due to carbon deposits, incorrect spark plugs, or overheating. Most people find the solution by moving up one grade at the pump. Deeper problems may require professional attention.

8. Piston Slap

On high-mileage vehicles, piston and cylinder wear may be so great that the piston no longer fits correctly. When the engine is cold, the piston may produce a rattling sound. The noise typically goes away once the engine reaches operating temperature and the piston expands. Piston slap is more of an annoyance than a true problem, and a permanent repair would require an overhaul with oversize pistons, costing up to thousands of dollars.

9. Rod Knock

Between the connecting rod and the crankshaft, a high-pressure film of engine oil, less than half the thickness of a human hair, keeps moving parts from contacting each other. Over time, due to wear, negligence, or abuse, that clearance may grow, leading to rod knock. It can eventually ruin the crankshaft, the connecting rod, or the entire block. Bearing replacement may resolve the problem, but a rebuild can be expensive.

10. Worn Drive Belt

Drive belts are a flexible construction of rubber over fiber and metal cords. Over many miles, as the belt starts to wear and degrade, it may fall apart. If the loose piece of an old drive belt is slapping around the engine at high speed, it can sound like an engine rattle. With the engine off, inspect the drive belts for tension, wear, and cracks. Replacement is an easy do-it-yourself job and will keep you from getting stranded.

11. Missing Insulation

Most modern engines are hidden under plastic covers and noise-blocking insulation. After years of use, wear, neglect, and degradation can result. To the untrained ear, noisy engine parts such as direct fuel injectors might sound obnoxious—they’re fine, though. Installing the factory noise-dampening materials will fix this problem.