Start with removing the external drive belts and running the engine if the noise is still there it will proberbly be a faulty timing belt idler or tensioner bearing. Check this and let me know what you find, if in doubt about doing this get a mechanic to do the check for you.
October, 26, 2011 AT 9:50 PM
The differential (final drive) bearings in the gearbox may be bad. When was the last time the gear oil has been changed?