When I have a problem I cannot diagnose, my first call is to Bruce at Bogart Engineering. He has helped me solve many issues by tracking down the sound. I persuaded Bruce to share his knowledge with us.
You’re driving home after a great day of off-roading and notice a new noise coming from somewhere in the Jeep. Dayum. Whattheheckisthat?
Well, as off-roaders we are always seeking new challenges. One of those challenges that’s a little unwelcome is the appearance of those new noises. You wonder what’s going to happen. Will the motor quit and leave me stranded? Will it suddenly steer itself off the highway?
So you pull off the road and open the hood. You look inside. Yep, the motor is still there. Good. You look under the vehicle. Nothing hanging down. Now what? You call a friend who’s a good mechanic.
What does it sound like? he asks.
You answer as intelligently as possible Clackety clackety clack.
He replies Sounds like the clacker to me. Better have it towed.
We don’t want this scene, do we? Well, I’m gonna tell you how to identify what’s happening in your vehicle and how to better communicate remotely with your buddy the mechanic.
The Source of a Noise
The source of a noise is identified by a combination of characteristics and a process of elimination: Speed (frequency)-under what circumstances does the speed of the noise increase and decrease?
- Does it speed up in direct relation to the road speed of the vehicle? Wheels, brakes, axles, differential, and driveshaft speeds are in direct relation to the speed of the vehicle. The driveshaft spins about four times faster than wheels and axles.
- Does it speed up and slow down depending on which gear the transmission is in? The engine (along with the accessories; alternator, water pump, power steering, and air conditioning) and transmission speeds are relative to the gear you’re in.
- Does it happen when the vehicle is sitting still with the motor running? If yes, then it can only be in the engine and accessories or transmission input. What happens when you depress the clutch, thus eliminating the transmission input? Further narrow it down by speed relative to the engine rpm. The rotation of the crankshaft and travel of the pistons define the speed of the engine. The valve train (tappets and rocker arms) runs at half the speed of the engine. The accessories vary according to pulley size.
Nature of Sound
- Is it a result of impact? Tapping or Banging would describe two solid metallic parts directly impacting each other. These are usually fairly easy to identify since there really aren’t many places that can happen. In the engine, the pistons can directly impact the head or crankshaft, and the rocker arms and associated valve train can have direct impact. Since the valve train operates at half the speed of the engine, you can hear a difference in speed and narrow it down. (tip: remove the oil filler cap in the valve cover and listen to the valves operate to hear half-engine speed) If it’s in the engine or accessories, it’ll happen with the vehicle sitting still and the motor running. If it happens while you’re driving, note if it’s aggravated by bumps and which corner hitting the bump aggravates it most.
- Does it squeal? Eek-eek-eek would describe a rotational noise that’s metal-on-metal. Usually it’s a bearing. The rate of change of the speed of the noise (as well as your directional hearing sense) will point you to the location of the bearing. So if it’s a road-speed eek-eek, it’s probably a wheel bearing. An eek-eek that varies with engine speed is probably an accessory bearing, often the alternator. A word of caution here: bearing eek-eeks often disappear when the bearing gets hot, so the disappearance of the eek-eek is not a good sign.
- Does it thunk? They’re usually isolated and not regular. If you can make it thunk by hitting a bump with a certain corner of the car, it’s a shock absorber. If you can make it thunk by hitting the throttle, it’s a motor or transmission mount. If it thunk-thunk-thunks at road speed, you’re probably losing a tire tread.
- Does it click? Chipped or broken gears click. Transmission or axle gears, depending on where you hear it coming from.
- Does it whine or even howl? Feed it or leave it home the next time you go out. Gears whine too. Usually the sound will change significantly on-throttle versus off-throttle. That’s never a good sound, but check yer freakin’ fluid level where the noise is coming from now! You’ve probably sprung a leak or gone dry. If you do it RIGHT NOW you might save those gears!
- Does it pop? Pops are the result of explosions. IE, valves in the engine open at the wrong time or not opening at all. Often accompanied by tapping because of a rocker arm problem. It can pop out the intake or exhaust end. Either way, it’s a pretty serious engine problem. UNLESS¦one of your joker friends swapped some spark plug wires while you weren’t looking. Death threats often uncover or cause such behavior.
- Does it vibrate? Everybody knows the vibration of a flat or out-of-balance tire. How about a vibration at four times that rate? That’s probably a driveshaft, bent or with a bad u-joint. Usually isn’t accompanied by noise, but we’ll include it here, and who could hear it anyway over all the other squeaks and rattles and flapping of the top?
- Place a long screwdriver, dowel, or mechanic’s stethoscope (get one on your next trip to Harbor Freight) against your ear and the various parts of a running engine to hear normal operating noises. Try the valve cover, crankcase, alternator, power steering pump, and air conditioner compressor. Caution: Don’t touch any moving parts or wiring!
- Tie a piece of nylon strap to a u-joint yoke just long enough to strike a frame rail, spring, exhaust pipe, or other part, and drive a bit. You’ll hear it slap against the part at about 4 times road speed. You may have to turn off the motor and coast to hear it. You can do this with a strap tied to a wheel spoke to hear road speed rotation. Keep it fairly slow or the strap will just coil up.
So to summarize, now you can call your mechanic and tell him: I’ve got a four-times road speed eek-eek in the rear. Whaddyathink? To which he can reply: Sounds like a bad pinion bearing. Check your gear lube. Drive slow with steady throttle. Watch for leakage at the rear of the driveshaft. If it gets worse, get it towed. Bring it to me in the morning.
Bruce Bogart, AKA Pappy, has been swappin’ lies around the campfire for over twenty years. He’s the inventor of the Plugzit and Starterita. After 45 years with cars of every description and ten trips across the Rubicon, he’s surely heard every bent and broken part imaginable. Although he’s become something of a recluse, he still enjoys hearing new lies and war stories at firstname.lastname@example.org.