Chasing down vehicle noises can be elusive work

Let's narrow the focus of all those noises to what Ford calls "Noise, Vibration and Harshness, (NVH)" because vibration and noise can be closely related. Noise is defined as an abnormal sound. Vibration is a trembling sensation that is usually tied to engine or road speed. Harshness is related to the way a vehicle rides.

One critical element of noise-hunting is to make sure that the noise you're chasing is the one the customer's complaining about, or else you might spend an afternoon fixing a problem that would annoy the heck out of you but wasn't even related to their concern, and then you have to start all over. Believe me, it can happen! I've done it more than once.

The Mustang Buzz

I drew a work order on a late 1990s Mustang when I was the drivability tech at a Ford dealer, and the customer concern was a buzzing noise that seemed to be originating from the instrument cluster area at highway speeds. In the days of mechanical speedometers, I would have gone in the direction of lubing the cable, but this unit was blessed with a cable-free electronic analog speedometer.

Finally, the discovery was made that the very same buzzing could be felt on the end of the owner's finger when he had his fingers just outside the driver's window.

Conclusion: The aerodynamic bug shield was creating a high frequency disturbance in the wind that was picked up by the windshield, amplified and projected downward to the speedometer area, which, by default, had been perceived as the originator.

Here's the progression: Typically, noises have an originator, a conductor and a reactor. In this case, the originator was the bug shield, the conductor was (by way of the air) the windshield and the reactor was the area around the speedometer head. The reactor is the noticeably affected component. With a vibration, the reactor might be a sun visor, the seat, the steering wheel, etc.

And don't forget about those funky air intake resonator chambers that prevent the induction system from hooting during heavy acceleration.

Whining, Whistling, Roaring and Rumbling

Sometimes the noise doesn't have a conductor or a reactor; it's just a noise. One example is a locked up viscous fan clutch on rear wheel drive vehicles. Not only does this concern make the engine very noisy, it robs the mill of a lot of horses. Check for a locked up fan clutch if it sounds like a plane on a runway and lacks power instead of a car on the freeway.

A 2000 Monte Carlo that belongs to one of our instructors at the college came to me with a nasty whining noise that the owner had determined to be in the transmission. He had paid for a tranny overhaul and had a lifetime warranty on that component. But when we drove the car, it didn't sound like a planetary gear set, which ordinarily is the source of transmission whining sounds.

Under the hood, the noise was really strong in the accessory drive area, and when we removed the serpentine belt, the noise was gone. Spinning every pulley, we found a rough and rumbling A/C clutch pulley bearing, which we promptly replaced.

A bad idler pulley bearing on a Windstar can burn you. I've heard a noise that I would have sworn was a power steering pump, and those bearings only made noise when heavily loaded, so they were not so obvious on a cursory hand spin. The irritating thing was that I replaced the PS pump on one of these before I discovered my mistake. That one was on a late 1990s Ford Windstar.

Wheel and axle bearing noises typically can be isolated by swerving gently from side to side. A basic rule of thumb is that when the car is swerved to the left, the right wheel and its bearing are loaded more heavily. Sometimes that diagnostic isn't accurate, though, so be careful. Chassis Ear or similar type tools are really handy for finding out exactly which wheels or bearings are the guilty parties on dynamically occurring noises. Noises that change with vehicle speed are driveshaft/axle or wheel related (bearings, brakes, etc.). I like to switch the car off and coast to take the engine out of the equation when I'm unsure, and with some noises, driving slowly next to a wall will amplify a noise that might be tough to hear otherwise.

When hunting noises in the engine compartment, a plain old $13 stethoscope is as handy as a shirt pocket. If you can't hear the noise with the probe, then remove the end of the tool and use it for a listening pipe. It's amazing what you can hear that way if you can neutralize the fan and its hot wind while you're investigating. A whistling noise I encountered on a new Ford Ranger changed pitch with throttle movement, and the throttle body gasket had already been replaced. So with a plain old piece of heater hose held to my ear and the other end tracking the noise, I found that the power brake booster check valve was split and whistling. The noise was originating a full two feet from the place where the attempted repairs had been done because moving the throttle changed the sound.

A 2003 Explorer that belonged to a college administrator was making a lot of bearing noise from the rear of the vehicle, and the Chassis Ear pinpointed the loudest noise (every noise sounds loud when amplified by the Ear) at the differential area. Victor, my then student, ripped the differential apart and found badly pitted bearings on the carrier. About $325 later, we had replaced all the bearings in the differential, and we felt sure the noise would be gone. But a lot of it was still there, and it turned out that the rear hub bearings (which carry the weight of the vehicle) were just as pitted as the bearings in the differential had been, which was mighty peculiar. This unit has CV axles. It was pretty amazing to find all those bad bearings whining and rumbling in unison. Was there a common denominator that made them all fail? Not that I could figure out!

Ricky is the manager at the city garage, and he beat himself up working on what sounded like a roaring or rumbling bearing noise on the right rear corner of a Crown Victoria police unit. Bearings and tires were replaced and moved around (tires can be really noisy even though they look just fine), and after a few days, the noise turned out to be a little American flag the patriotic policeman had mounted on the left rear fender flapping aggressively at road speed. The noise was telegraphing down through the fender, into some cables that were feeding the tape machines and other police hardware, which were in turn transferring the noise into the trunk shelf in that corner of the car, creating a bearing-like noise.

Popping and Clicking

Struts are famous for making odd rackets, particularly if the bearing at the top is worn out.

At the VW dealership years ago, I found a noisy rear strut rattling and popping on a Jetta, but I could find no problem with the strut when I removed and disassembled it. Because the car was under warranty, I ordered all the parts to assemble a new strut, and the problem was gone.

Ford pickups in the mid- to late-90s vintage can have loose rivets on some of their cross members that will cause scary popping or snapping noises on uneven ground. It's a good idea to lubricate the steering stops on just about any vehicle that has them, or else the customer might hear strange squeaking noises when the wheels are turned to the stops while driving on uneven ground.

On a few occasions when I have let students or tire shops change the oil in my Jeep Cherokee, I've found that my canister purge solenoid would be out of place to the point that it was in heavy contact with the A/C suction line. When the PCM decides to purge the canister (which is likely to happen anytime the engine is running), the odd thumping sound of the canister purge valve is carried into the evaporator case and the bulkhead, sounding for all the world like an engine knock.

Likewise, I've seen Mustangs where engine noise was being transmitted into the body because an A/C line was making heavy contact with a shock tower.

Scrubbing

Here's a note: Scrubbing or other type noises that seem brake related can be a harbinger of doom if ignored, so do a thorough brake inspection when confronted by such issues, lest you overlook something safety-serious and come to ruin.

One fellow had a scrub-scrub noise that was really loud on his Ford pickup, but, like the Jeep, the scrubbing would go away as soon as he applied the brakes.

The brake pads had been replaced, the rotors had first been turned and then replaced, and finally more new pads and rotors were installed. But the problem always returned after a couple of days.

Since the truck was 4-wheel drive, a student of mine came up with the idea that we should raise the truck on the lift, put it in 4-wheel drive and watch the noisy brake assembly with the tire and wheel removed.

We did, and we saw the rotor exhibiting some pretty remarkable lateral runout. Because the rotors were new, we removed the caliper and rotors, checked the hub with a dial indicator, and found about .015-inch runout. After replacing the $180 hub, the noise was gone and never returned.

Conclusion

NVH diagnosis is a necessary part of what we do, and full service shops have to take it seriously. It can be tough, but with some special tools and the right amount of bulldog determination, we can turn a noise-conscious customer into one of our best word-of-mouth advertisers, particularly if other shops have thrown in the towel.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. He is an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail him at [email protected]