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BMW Noise Troubleshooting:
Troubleshooting Noises (Is Your BMW Trying to Tell You Something?)
Troubleshooting those weird noises coming from your BMW can help you determine needed repairs. It’s important that you listen to what your car is trying to tell you.
Although many people still use the old broomstick-held-to-the-ear
method of zeroing in on noises, the best method these days is a
stethoscope. Indispensable in finding the source of a sound, it’s also
a lot easier to place it where you want it than the clunky end of a
broomstick.
Here are a few general guidelines to what your BMW’s noises might mean:
BANG: A sharp, startling sound, like a rifle shot,
means you’re dealing with the dreaded backfire. You’ll probably be able
to trace this to something that’s causing a rich air/fuel mixture.
In the past you might have zeroed in on a heavy carb float, but
today think about faulty signals from coolant temp or O2 sensors. The
catalytic converter may also be damaged.
Another possibility is a clogged monolithic converter blowing
through. This will only occur once and will be accompanied by an
amazing increase in power. If your car has air injection, perhaps the
diverter valve is no longer diverting.
BOOM: A hollow, low-frequency sound/sensation, this
makes you feel as if you’re riding inside a metal drum and the
atmospheric pressure is rapidly changing between positive and negative.
On rear wheel-drive cars, check out the driveshaft and its u-joints
because if it’s spinning out-of-true, it will cause waves that push up
on the floor of your car.
BUZZ: An annoying “bzzzzzzzzz” sound, like a
trapped insect, can usually be traced to unfortunate positioning of
interior trim parts. Have somebody else drive while you press, pry and
pound on every likely spot.
CHIRP: This sounds like birds are nesting under
your hood. You can probably blame a maladjusted or misaligned belt, but
don’t ignore the idler pulley. Or, it could just be your tires when you
hit second gear.
CLANG or CLANK: This sound couldn’t possibly be
emitted by any light, flimsy parts. It’s coming from a heavy, essential
component, such as a set of gears. A good example is the sound a bad
rear axle pinion bearing makes when you drop the transmission into
Drive, then Reverse.
CLICK or CLACK: This sounds like 007 working the slide of his Beretta automatic. When in an engine, it’s typically repeated rhythmically.
With OHV, perhaps a stuck lifter is allowing clearance in the
pushrod/rocker valve, or maybe a solid lifter is just out of
adjustment. On carbureted cars, check out the fuel pump before you
start opening up the motor.
When emanating from the nether regions of the front end during a turn, this sound may be traced to an outboard CV joint.
CLUNK: A heavy bumping sound, softer than a clang,
usually indicates you should look at suspension bushings, including
shock or strut mounts. Or how about a loose strut gland nut?
FLAPPING: If it’s not due to a colony of bats under
the hood, maybe a belt’s coming apart. Fan interference is another
possibility. Regardless, this is a visual inspection sort of thing.
GRINDING: A horrible, torturous sound, like a bad
dentist would make while working with obsolete equipment, means
something’s going awry - and fast.
If it occurs when the brakes are applied, either the linings are
gone or you’ve got one of those unpleasantly-aggressive friction
material formulas that tend to eat rotors.
GROAN: Something’s dry, probably a suspension
component. If it’s metal, it’s going to break really soon. If it’s
rubber, try some silicone lube.
GRUNT: Again, a dry joint somewhere in the
underpinnings is likely. If it’s in the stoppers, suspect rear drum
shoes contaminated with brake fluid or gear lube from a defunct axle
seal.
HISS: If it’s continuous and changes with rpms, it
may be normal belt noise. Otherwise, a slow leak in the cooling system
is likely. A black light will help you find this.
HUM: We don’t mean what the radio does between
stations, but the noise a differential or wheel bearing makes. If it
responds to acceleration/deceleration, suspect the differential. Then
look into the bearings. Unfortunately, it’s often very difficult to
tell which side (or even which end) the hum’s coming from.
KNOCK: Like knuckles on a wooden door, this sound
is deep and hollow. Often it’s a warning that something important (and
expensive) is about to let go.
It’s unfortunate that a loose pin sounds pretty much the same as a
defunct rod bearing, but with a little patience you should be able to
determine what’s at fault.
First, check idle oil pressure even if you have to screw in a
mechanical gauge. If it’s low, you can bias your decision toward
bearings.
Next, listen with your stethoscope. A rod bearing makes more noise
at the oil pan than elsewhere, and a wrist pin more racket up on the
water jacket. Hold RPMs at 2500, jerk the throttle open and let it snap
closed. This will accentuate rod knock, whereas pin noise won’t change
very much.
Now’s the time to starting shorting out cylinders. A bad pin will quiet down, but a rod knock will double its cadence.
Finally, you can pull the pan for a visual inspection. If the bearings are good, you know you’ve got a pin problem.
PING: Sort of like little ball bearings being
poured on a tin roof, this sound is detonation (aka spark knock) - a
phenomenon in which the air/fuel charge explodes violently instead of
burning smoothly.
There are many potential causes here from clogged EGR passages and
overheating to excessive spark advance and, with spark knock
suppression, a defunct detonation sensor. Hook up your timing light
then tap on the engine near the sensor to see if the spark retards.
POP: This sounds like a shotgun being fired through a mattress. It usually means the engine’s coughing back through the intake.
A sticking or leaking valve is a distinct possibility, as is jumped valve timing, particularly with a belt-driven OHC.
Then there’s ignition, which may be firing way too early due to a
twisted distributor, cap/rotor/wire problems, a faulty position sensor
or a breakdown in the module.
Also, if your BMW’s running quite lean, opening the throttle to lots of cold air can induce this reaction.
RATTLE: They didn’t coin the term “rattle trap” for
nothing, you know. People have been fighting this annoying noise since
the automobile was invented.
Thanks to plastics, better rubbers and more highly engineered
fasteners, rattles are less prevalent than they once were. But you’ll
still get them, usually in the undercarriage somewhere. Likely culprits
include exhaust system parts, calipers or loose brake pads.
ROAR: If it’s not something obvious like a blown exhaust system, maybe the transmission is never shifting into high or overdrive.
With a manual transmission, the clutch might be slipping. Fan
clutches usually fail by never engaging, not the opposite, but it’s
still a possibility.
If it’s general road noise, you could switch to less aggressive tires or add undercoating to your BMW.
RUMBLE: While a pleasant enough throaty sound when
it’s from a free-flowing exhaust system, it can easily cross over into
the unacceptable sound range. But don’t choke the power down with an
overly restrictive cheap muffler. For tire and road noise, see “ROAR”.
SCRAPING: Something like “jeet-jeet-jeet-jeet” that
speeds up as the car gathers speed probably means an object of one sort
or another is contacting the driveshaft, possibly an exhaust shield or
hanger or the parking brake cable. Your brake system, especially drum
hardware, is also a distinct possibility.
SCREECH: “SQUEAL” taken to the max. See “SQUEAL”.
SIZZLING: Like the sound of bacon frying, this is
usually only audible with the engine off. Oil may be leaking onto the
exhaust manifold or a minor coolant seepage may be occurring.
SQUEAL: This sound is usually related to brakes and
belts. On the former, maybe you’re down to the pad wear indicators. Or
the discs and semi-metallic linings aren’t getting along due to poor
rotor finishing or washing, an assembly error, a troublesome friction
formula or the like. Squealing is certainly common in disc brakes, but
clunking can also occur on initial application if the shoes are loosely
mounted.
In the case of belts, check if they are loose, worn or contaminated.
TAP: Much the same as a click, sort of like beating
on the intake manifold with a screwdriver blade, this is usually
valvetrain-related. Think about stuck lifters or an adjustment that
provides too much lash.
WHINE: Not what an impatient 3-year old does but
just as annoying. This is a hard one to pin down, but it’s apt to come
from worn ball or roller bearings, mismatched gears, too light a lube
in a manual gearbox (ATF, maybe?) or alternator bushings getting ready
to go.
WHIR: The sound made by happy mechanicals. It’s one of the few noises you probably shouldn’t worry about.
WHISTLE: Usually occurring at higher speeds, it’s
probably wind noise. But do double check if the latches and tumblehome
are properly adjusted. Are the body gaskets in good shape?
Don’t Forget:
Repair articles are added regularly.
Come back often to check for new maintenance topics.
These repair tips are designed only as a starting point.
Please seek the assistance of a professional BMW mechanic
for all repair problems beyond your capabilities.
Regards,
Saftari - Kemangnet.com
Sumber: www.saft7.com

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