Troubleshooting those weird noises coming from your Mercedes 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 Mercedes' 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 Mercedes' 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 Mercedes.
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?