My Ford Tempo has begun to make a loud grating noise from the engine – intermittently while at idle, and as I begin to accelerate. Could you please offer a suggestion as to how I might search for the cause? Windel Newby,

San Jose, Calif.

This is a challenge, as NVH (noise/vibration/harshness) concerns can be tough enough to solve even when one can listen, poke and prod. Since the noise comes and goes at times, my hunch is that the cause is external to the engine, and is vibration- or load-influenced.

When at idle, does the noise surface as the steering wheel is turned, the air conditioner or defroster is engaged or the radiator cooling fan operates?

If so the cause might be a faulty serpentine belt idler pulley, loose accessory drive component or cracked attaching bracket. The above situations place a greater load on the drive belt and related hardware, possibly bringing the grating/vibration to a noticeable level.

Assuming the noise occurs regularly – you’re able to induce it within a moment or two – I’d note the routing of the serpentine drive belt’s many twists and turns, then remove it. Then, operate the engine for no more than two minutes and check to see if the noise abates. This brief time of operation is essential, as the water pump will not be circulating coolant throughout the engine.

If the grating noise is no longer detected, turn the engine off and put the key in your pocket. Then, try rotating each belt-driven accessory and both idler pulleys by hand. If roughness or looseness is evident, replacement might be necessary. Also, give each part a strong shake/wiggle, checking for possible bracket or mounting problems.

If the belt/accessory prognosis proves moot, next check items attached to the engine, and the engine mounts. Check for metal items – such as an air conditioning tube rubbing against the firewall, loose brackets retaining the radiator, A/C condenser, transmission dipstick tube, etc. There are dozens of possibilities.

A faulty engine or transmission mount could allow metal-to-metal contact as the engine/transmission wiggles during operation.

Carefully applying force to the engine with a large pry bar or pipe can help to detect mount problems. The noise might change or stop as the engine is loaded in a particular direction.

Another possible cause of such a noise is a cracked flexplate (flywheel). Under certain loads, or the absence of them, a knocking/grating sound might occur.

Visual access will be difficult. Check for rust-colored spider-web-like marks near the torque-converter mounting bolts and center hub.

If tempted to poke around while the engine is running, be extremely careful to keep your hands and clothing well way from the belt area, and away from hot items such as the exhaust manifold.

Also, accidental contact with a faulty spark plug wire can cause a nasty shock and can cause one to lose motor control and come into unintended contact with a dangerous component.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.

Write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190; or e-mail to under-the-hoodjuno.com. He cannot make personal replies.)

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AP-NY-11-04-05 2038EST