* Low oil pressure when cold: While the oil was *changed*, there's a possibility of an issue with the oil pump itself, a blocked oil filter, or even low oil level (despite a recent change - double check this!). Low oil pressure when cold means insufficient lubrication to the valve train, leading to the noise. This is a serious issue that needs immediate attention.
* Valve lifter issues (hydraulic lifters): These are common culprits in this type of situation. Hydraulic lifters rely on oil pressure to compensate for wear and maintain a consistent valve clearance. If they're worn, sticking, or not properly filling with oil when cold, they'll cause a ticking or tapping noise. Once the oil warms and thins, the noise often subsides.
* Worn valve guides or rocker arms: Less likely, but possible, wear in these components could cause a noise that is temperature-dependent.
* Incorrect oil viscosity: While less probable given a GM Italy service, it's possible the wrong oil viscosity was used. A thicker oil takes longer to circulate effectively when cold, exacerbating the noise.
What to do:
1. Return to GM Italy: Since the oil change was recently performed by them, this is your first port of call. Explain the problem clearly and insist on a thorough investigation. They should be able to diagnose the issue under warranty (depending on the age and mileage of the vehicle and specific warranty terms).
2. Check the oil level: Make absolutely sure the oil level is correct. Even a slightly low level can exacerbate the problem.
3. Obtain a second opinion (if necessary): If you're not satisfied with the GM Italy response, get a second opinion from a reputable independent mechanic specializing in GM vehicles.
Important Note: Ignoring this noise is risky. Continuing to drive the vehicle with a noisy valve train could lead to further damage, potentially causing a catastrophic engine failure. Get it checked out promptly.
