I eventually found the noise (sounds like a tambourine at idle) comes from the belt tensioner. It seems right below the alternator, and hard to reach. I want to replace it myself, and placed hold of the Haynes repair manual from library. Well, I still wonder if it's an easy work to do? Even the noise is quite annoying, I'd rather just drive the van if the belt tensioner will keep working.
Advise please, thanks.
Simon