is made up of many moving parts that have close tolerances. These parts are
lubricated by pressurized motor oil provided by the oil pump which is
located inside the lower part of the engine inside the oil pan. It is normal
for an engine to make a slight rumble or roar while running but if you hear a
distinct rattling noise there is a problem.
What Goes Wrong?
Many spring steel and metal bearing components make up the inner workings and
several external accessories of an engine. These metals are subject to many
cycles of being hot and cold which can fatigue the material causing them to
crack or break. Surrounding an engine is also a variety of metal plates that
support accessories which must be
driven by the engine such as the alternator to supply electrical power to
devices such as the fuel pump and lighting systems. If a mounting bolt to one of
these supporting brackets is missing or left loose at the factory or during a
repair it can cause additional strain on the metal creating a break which will
make a rattling sound.
What does it Cost?
The cost of fixing a rattling sounds will vary greatly for example; a loose
or missing bolt on an exhaust system heat shield would cost about $20.00 to $25.00
(US) if you took the car into a shop. If an internal engine part has failed the
bill could be a bit larger if a timing belt or chain tensioner or guide needed
to be replaced which can cost between $400.00 and $800.00 (US). Your best
defense again unexpected surprises is to do a little investigation and see if it
is something you can fix yourself or have done at a shop.
The full video of this repair is at the end of this guide.1. Low Engine Oil
Motor oil is used to lubricate the inner working of the engine. This oil
needs to be checked from time to time because it slowly gets burned by the
engine. When the oil level becomes low it will allow the oil pump to scavenge
air and mix it with the oil causing friction between moving parts which can
produce a rattling noise. Shut the engine off and then locate the oil level dip
stick which is usually designated by a yellow handle. Remove the dip stick and
wipe it off with a paper towel and reinsert it. Then pull the stick out to
observe the oil level it should be between the two marks or dots on the stick.
If no or low oil levels are observed then
add oil until the level is correct. The space between the marks represents 1
Watch the Video!
Here is a video of a complete oil change where our mechanic checks and adds
2. Bad Serpentine Belt Tensioner
A belt tensioner is designed to hold pressure on the belt so it does not slip on
the drive or accessory pulleys. This part is spring loaded and when its internal
spring breaks or when the tensioner pivot wears out it can cause a loss of
pressure which will cause the belt to slip and then quickly grab causing a
rattling sound. While the engine is off reach in and test the tension of the
belt. If the belt is loose or has little pressure holding the belt then
tensioner has failed and needed replacement. You can also start the engine
and observe the tensioner in action. If there is excessive vibration suspect the
belt has gone bad and is in need of replacement.
Watch the Video!3. Loose Exhaust Heat Shield
The exhaust system in the car uses heat shields over the manifolds, catalytic
converters and exhaust tubing to keep heat from penetrating the floor of the
car and causing damage. These shields are made of thin metal and when they come loose or break they
can make a rattling sound while the engine is running. Some of these shields are accessible from the engine compartment while other times you will need to
jack up the car and get under it to check out the shields.
4. Bad Water Pump
The engine's water pump is driven by the serpentine belt via the pump's drive
pulley which is supported by the main shaft bearing of the pump housing. When these
bearings fail it can cause the pump shaft to wonder which produces a rattling
sound. This may or may not be accompanied by a
To check for this problem have the engine off and work the water pump pulley
back and forth while looking for play. Excessive movement is an indication the
has failed and needs replacement.
Watch the Video!5. Failed Idler Pulley
An engine's idler pulley is used to support the serpentine belt between the tensioner,
drive pulley and the accessories. This part is basically a pulley with a bearing
that is mounted with a through bolt. This pulley can be grooved or smooth
depending on which side of the belt it is supporting. To check if this is your
problem, remove the serpentine belt and spin the idler pulley to listing for a
ratting sound which means the pulley has gone bad and needs replacement.
Watch the Video!
6. Cracked Engine Accessory Bracket
A mounting bracket is used to support the various accessories such as the power steering pump, air conditioner compressor
and alternator. If the bolt becomes loose or the bracket breaks
it can cause a rattling noise. These brackets can be both cast or stamped steel.
Inspect the brackets to make sure they have all of their mounting bolts installed
and look for cracks somewhere on bracket. On steel brackets the crack will be accompanied by a rust
colored dust at the crack to make it easier to see. If a failure is observed the
bracket must be replaced or welded.
7. Worn Timing Belt
A timing belt or chain tensioner is
used to rotate the camshaft(s) that control intake and exhaust valve operation.
If this belt, chain or tensioner becomes loose or is failing it can cause a rattle
noise that follows the engine RPM's. To check for this condition remove the timing belt
or chain cover and use a flashlight to check for excess play or parts of the
timing belt that have come off.
8. Broken Harmonic Balancer
A harmonic balancer is used to help silence engine vibration by utilizing a
counter balancing weight supported by a rubber ring which is located at the
front of the crankshaft. This ring can become dislodged because the rubber
insulator has failed. This will allow the outer ring to bounce and jump on the
rotating hub which can produce a rattling noise that changes with engine speed.
To check for this condition remove the serpentine belt and inspect the harmonic
If the balancer has come apart it must be replaced. You can get the job done by
watching the video below.
Watch the Video!
9. Broken Fan Clutch
On rear wheel drive cars a
is attached to the front of the engine to help cool the radiator. If this
component fails it can allow the main shaft of the clutch fan to have excessive
clearance allowing the main housing to have "play" while producing a rattling
sound. To check for this failure (engine off) grasp the fan blade and check for
excessive movement. Usually this condition will be accompanied by the engine
running hot or overheating but not always. It should turn in either direction as
Watch the Video!
10. Bad Starter
starter motor is used to crank the engine over. This unit has a bendix that
is used to engage the starter when in use and then disengage the starter when
not in use. The bendix has a return spring that can break allowing the starter
bendix gear to contact the flywheel creating a rattling sound which occurs under
acceleration. To check for this problem the starter must be removed
to see if the bendix gear is not being returned into the starter motor. If the
spring is found to be broken the
starter motor must be replaced.
Watch the Video!
11. Cracked Flex Plate
On vehicles equipped with an automatic transmission a flex
plate is used to couple the transmission with the engine. This plate also
supplies the starter with a ring gear which is used to start the engine. If this
flex plate is cracked or is loose it can produce a rattling noise. The
transmission will need to be removed to check the flex plate in most cases. Look for rust colored
powder either at the crankshaft bolts or the torque converter mounting bolts. Also look for obvious cracks
and missing or loose bolts.
12. Engine Ping
Engine ping occurs mostly in older vehicles and is the result of a detonation
inside the combustion chamber resulting in an uncontrolled explosion (ping).
This ping has been described as an engine rattle. Most engines are equipped with a knock sensor which
controls this condition by signaling the computer to retard the ignition timing,
but if the fuel octane levels are too low it will cause the engine to ping anyway.
If the problem persists the engine could have excessive carbon build up and a de-carbonization
service is needed.
Watch the Video!
Here is how our mechanic would go about looking for an engine rattle.
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