Car Problems With a Rattling Sound When Acceleratingby Robert Moore
Rattling noise during acceleration can be caused by several different but common failures, including loose heat shields, bad belt pulleys and ignition pinging. Regardless of the fault, you must identify and correct the problem quickly -- otherwise your problem will get worse.
Heat shields are a common source of rattling. Usually made from aluminum, they provide a thermo-reflective barrier between your car’s exhaust system and other parts of the vehicle. These shields prevent heat transfer into the cabin of the vehicle, the fuel lines and even the gas tank. Regardless of what they protect, when they come loose, they can make some scary rattles when the engine is under load.
Never attempt to inspect or repair the exhaust system when the engine is running or has been recently running. Portions of the exhaust can remain hot for hours after an engine has been shut off.
Inspect your exhaust system from front to back. Heat shields are flexible but are normally mounted so that they cannot move or shift position. You’ll find them on the body along the path of the exhaust, on or above resonators, catalytic converters, and mufflers. Inspect each heat shield for damage, looseness or wear from contact with another metal surface. Bend or shape any of them, as needed, to prevent metal-on-metal contact. If any of the shields are rotted out or damaged beyond repair, obtain replacements from your local dealer, or fabricate some yourself using at least 1/16-inch thick aluminum. If you fabricate your own, weld or mount the shield so that there is a small air gap between the exhaust and the shield, as well as between the shield and whatever it protects.
Your car’s engine idles at a relatively low speed -- somewhere around 500 to 900 rpm, depending on make and model. As engine speed increases so does the speed of every pulley in the accessory drive system. Loose or even slightly bent pulleys can sound like death at higher speeds, as metal-on-metal contact occurs thousands of times per minute.
The easiest way to determine if a bad pulley is the cause of your rattling, is to visually inspect the belt as the engine is running, and when the engine is turned off.
Do not let your clothing, hair or jewelry come into contact with the belt while the engine is running.
With the hood open, you may be able to hear the noise at idle and pinpoint the general location. When the engine is running, a loose pulley will cause the belt to wobble back and forth a little bit. The belt itself will also exhibit damage in the form of frayed sides from being pulled against the sides of the other pulleys. If you think the rattle is in the accessory drive system, remove the belt and spin each pulley by hand. Try wiggling the pulleys to see if you get any movement. Replace any pulley that is loose or wobbles. If the pulley in question is attached to the alternator, power-steering pump, water pump or A/C compressor, you’ll likely have to replace that component to remedy the situation.
Ignition pinging is often interpreted as a rattle at first because of the metal-on-metal noise it creates, and it is commonly heard only on acceleration. Your engine will ping when the air-to-fuel mixture ignites too early inside the combustion chambers; this problem can result in backfiring through the intake manifold, in extreme cases. If you own a newer vehicle, ignition pinging occurs if you use fuel with too low of an octane rating or if you got stale gas on your last fill. Use a higher octane fuel and see if that remedies the noise on acceleration. If you have an older vehicle with a distributor ignition system, check your engine's spark timing. If the ignition timing is off, that is likely the source of the pre-ignition.
It's Not Always a Cheap Fix
Other sources of rattle can be quite serious, and you may hear even hear it at idle from the engine bay. Don’t confuse excessive rocker arm chatter for a harmless rattle. If the rattling noise sounds like it is coming from either side of the engine, near the top, you just might have some work to do. Place the handle of a screwdriver up to your ear, and carefully touch the tip of it to each valve cover with the engine running. If you can hear the rattle or chatter through the screwdriver, then you know what side needs work. It could be as simple as adjusting valve lash, or you may need to replace one or more of the rocker arms. If you are inexperienced or uneasy about taking on a job like this, it is best to leave this repair to professionals.
Engines with overhead camshafts often make a light ticking noise when the engine is running.
- link How heatshields work