It should be engine related and there are many possibilities.
If it is internal, it could be from the excessive clearance of the valve rocker arms, either due to wear of the arms or out of adjustment.
External noises could be from faulty
2. A/C compressor bearing
3. Idler pulley bearings
4. Water pump.
You need to check and confirm where the source is from. Use a rubber hose as hearing aid and put it to the area which is emitting the noise and see where it is loudest. That would help you narrow down the source.
Saturday, February 7th, 2009 AT 6:20 AM