Loose Main Bearing
A loose main bearing is indicated by a powerful but dull thud or knock when the engine is pulling. If all main bearings are loose a noticeable clatter will be audible.
The thud occurs regularly every other revolution. The knock can be confirmed by shorting spark plugs on cylinders adjacent to the bearing. Knock will disappear or be less when plugs are shorted. This test should be made at a fast idle equivalent to 15 mph in high gear. If bearing is not quite loose enough to produce a knock by itself, the bearing may knock if oil is too thin or if there is no oil at the bearing.
A thud or click which is usually irregular. To test, idle the engine at about 20 mph and shut off the ignition. If thud is heard, the flywheel may be loose.
Loose Rod Bearing
A metallic knock which is usually loudest at about 30 mph with throttle closed. Knock can be reduced or even eliminated by shorting spark plug. If bearing is not loose enough to produce a knock by itself, the bearing may knock if oil is too thin or if there is no oil at the bearing.
Piston pin, piston and connecting rod noises are difficult to tell apart.
A loose piston pin causes a sharp double knock which is usually heard when engine is idling. Severity of knock should increase when spa/k plug to this cylinder is short-circuited. However, on some engines the knock becomes more noticeable at 25 to 35 mph on the road.
Piston pin rubs against cylinder wall, caused by lock screw being loose or snap ring broken.
Piston & Rings
The malfunctioning of a hydraulic valve lifter is almost always accompanied by a clicking or tapping noise. More or less hydraulic lifter noise may be expected when the engine is cold but if lifters are functioning properly the noise should disappear when the engine warms up.
If all or nearly all lifters are noisy, they may be stuck because of dirty or gummy oil.
If all lifters are noisy, oil pressure to them may be inadequate. Foaming oil may also cause this trouble. If oil foams there will be bubbles on the oil level dipstick. Foaming may be caused by water in the oil or by too high an oil level or by a very low oil level.
If the hydraulic plungers require an initial adjustment, they will be noisy if this adjustment is incorrect.
If one lifter is noisy the cause may be:
If ball valve leaks, clean plunger in special solvent such as acetone and reinstall. Too often, plungers are condemned as faulty when all they need is a thorough cleaning.
Gum and dirty oil are the most common causes of hydraulic valve lifter trouble. Engine oil must be free of dirt. Select a standard brand of engine oil and use no other. Mixing up one standard brand with another may cause gummy oil and sticking plungers. Do not use any special oils unless recommended by the car manufacturer and change oil filter or element at recommended intervals.
Loose Engine Mountings
Occasional thud with car in operation. Most likely to be noticed at the moment the throttle is opened or closed.
Excessive Crankshaft End Play
A rather sharp rap which occurs at idling speed but may be heard at higher speeds also. The noise should disappear when clutch is disengaged.