4 Things That Cause A Knocking Noise From Your Car: Capitol Subaru Auto Care Articles

 

4. Rod Knock

Deep inside your engine, pistons travel up and down in their cylinders, pushing down and spinning the crankshaft. This is what ultimately drives your car. Parts called rod bearings mate the piston rods to the crankshaft. If these bearings fail, a gap can form between the piston rods and the crankshaft. As the piston shifts around due to this gap, it'll rattle back and forth, creating a percussive knocking noise from under the hood. Piston rod bearings that knock will need to be replaced -- a costly and time-intensive service due to their location deep in the heart of the engine. To avoid having to replace piston bearings on your vehicle, the most important thing you can do is replace the oil in the engine regularly. Fresh engine oil conditions your vehicle's seals and bearings so they last longer and perform their best.

 

3. Detonation Knock

Another knocking noise from the engine, sometimes described as a "ping" noise, is called detonation knock or pre-ignition knock. This noise is caused by pockets of the air-fuel mixture in an engine's combustion chamber detonating spontaneously, before the spark plug fires its spark. Most commonly, this is caused by using fuel with an insufficient octane rating for your car. If your vehicle requires premium fuel (such as a turbocharged Subaru WRX), this kind of knock can be caused by accidentally filling up the vehicle with regular grade fuel.

But if that's not the case, we'll want to check your engine for a variety of problems, such as a compression leak, incorrect timing or even a failed knock sensor. A bad knock sensor can actually induce the condition it's designed to detect. How's that for irony?

 

2. Valve Rattle

The latest engine designs boast sophisticated valvetrains featuring complex variable valve timing and variable valve lift systems. They make your engine run more efficiently and generate more power in a variety of different conditions. However, when these systems develop a problem, you might hear a tapping noise from the engine -- a quieter, lighter sound than you would get from rod bearing knock. Depending on what kind of engine your vehicle has, this sound can be caused by bad lifters, bent pushrods, worn valve seals or insufficient oil lubrication in the valvetrain. You'll want a technician to make sure you're using the right oil for your engine and that you don't need any valvetrain components replaced.