It seems some belt is making the noise when idling. Any idea? Which one should I check for crack?

 

Its a serpentine belt, correct? yes, check for cracks, if it doesnt look shiny and new, replace it. Otherwise, crank down on the tensioner.

if its a vg motor I understand that it controls timing too. Hopefully somoeone else will tell you what to do to make sure timing is back to spec.

 

If it's squealy, it's probably one of the external alternator / aircon etc belts, rather than the timing belt- I doubt that would make much noise. It'll be either OK or broken, in which case, you'll be climbing trees to collect your valves.

Back to the easier belts. Run the engine for a bit with squealing noises. Switch off. See which belt burns your (or better still, your assistant's) hand the worst. That'll be your bwoy right there.

Visual checks are easy. Do the belts look new, or old and crappy? Shiny on the inside is bad.