11 Engine Rattles and How to Fix Them

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The modern internal combustion engine is a complex ballet of hundreds of parts, designed to convert fuel energy into motion. Looking under the hood, not much is visible, much less heard. An engine rattle is usually a sign that something has gone wrong – leaks, smells, poor performance, and the check engine light are others. The underlying cause of an engine rattle could be inconsequential or it could mean something is close to failure. Here are 11 engine rattles, how severe they are, and how to fix them.

1. Broken Belt Tensioner or Chain Tensioner

Drive belts, timing belts, and timing chains need to have a certain measure 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 could bounce, leading to an engine rattle. Replacing the broken tensioner fixes the problem, but putting it off could leave you stranded (broken drive belt) or with severe engine damage (broken timing belt or chain).

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. In the case of ceramic-based catalysts, thermal shock or an impact could cause the matrix to crack. If a piece breaks off, you could 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, leading to a rattle. On its own, a collapsed lifter may not cause any damage, though it might cause a cylinder misfire. Replacing the lifter and adjusting the valve clearance will fix this rattle.

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 may appear around the bolts at the crankshaft, sometimes completely separating at that point. It can be hard to pinpoint this kind of failure, but it could leave you stranded. Flex plate diagnosis and replacement requires transmission removal, which can be expensive.

5. Low Oil Pressure

Oil pressure runs components like variable valve timing (VVT) and hydraulic valve lifters. With insufficient oil pressure, these parts may not function, rattling in the valves or VVT drivers. Check 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 (expensive) or emissions are critical (irresponsible). Otherwise, you may have another problem in the oil pump system.

6. Rusted-Out Heat Shield

Passenger vehicles are averaging nearly 12 years old across the US, which means many cars are experiencing problems their ancestors never experienced, such as debilitating corrosion. In some places, such as on the catalytic converter or muffler, heat shields protect the body, transmission, or other component from exhaust heat. On and around the exhaust system, heat accelerates corrosion. A rusted-out heat shield could fall off as likely as get stuck, sounding like an engine rattle. Heat shields can sometimes be removed cheaply, but replacement is a better idea.

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 and suddenly boost the pressure. This is usually caused by low-octane fuel in a high-compression engine, but can also be due to carbon deposits, incorrect spark plugs, or overheating. Most people find the solution 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 fits sloppily. When the engine is cold and the piston is smaller, this sloppiness exhibits itself as an engine rattle. The noise typically goes away once the engine reaches operating temperature and the piston expands. This is more of an annoyance, though a permanent repair would require an overhaul with oversize pistons, perhaps costing thousands of dollars.

9. Rod Knock

Between the connecting rod and 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 could eventually ruin the crankshaft, 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 a loose piece of an old drive belt is slapping around the engine at high speed, this could sound like an engine rattle. With the engine off, inspect the drive belts for tension, wear, and cracks. Replacement is an easy DIY job and will keep you from being stranded.

11. Missing Insulation

Most modern engines are hidden under plastic covers and noise-blocking insulation. Over years of maintenance and repair, wear, neglect, and degradation may result in these covers missing or poorly installed. To the untrained ear, noisy engine parts, such as direct fuel injectors, might sound obnoxious – they’re fine. Installing the factory noise-damping materials will fix this problem.

Take Care of Engine Rattles

If you’re hearing a rattle under the hood, it could be benign or a sign of impending failure. Either way, check it out and see if there’s something you can do about it. Check with a tech-savvy friend or take it to your local trusted automobile repair technician. Fixing an engine rattle right away could save you thousands of dollars in collateral damage, save money on gas, and help you regain your sanity.