by Alain Hoffmann
Noises - Main Bearings
You will probably never hear this one if you're not an engine builder or driving old, worn out rigs all the time. But it's nice to know what it sounds like anyway. It's a strange, metallic noise, not sharp and not as rapid as the valve train noise. It's better at speed and worse when you lug the engine or at idle. Usually indicates a worn main bearing.
So what to do? Sadly you can't pull of the oil sump and listen directly to the bearing. It would be nice but, you know, it's a cruel world. The only way to diagnose an main bearing's play is by measuring it. Take off the bearing cap, but some plastigage (an deformable strip of specified thickness) in, tighten up, slacken again, take plastigage out and compare it to the printed strip on the box it came in. Quite easy and safe - just takes a day's work. But at least you MAY leave the engine in.
But this noise can also come from crankshaft end play. It's sharper and more irregular but as you probably heard neither you won't be able to tell anyway. It vanishes if you put load on the crank like when you push the clutch pedal. Luckily I hadn't yet to check that so I must refer you to the official bible book (aka Factory Repair Manual).