Q&A

 Master

The noise is very hard to guess at without hearing it first hand but Toyota Previas do have one important thing that commonly goes wrong that could cause this problem.
Your van has the engine mounted under the front seats and uses a drive shaft (with flexible couplings that fail) to drive engine accessories (alternator, air conditioning compressor ect..) at the front of the vehicle behind the grill/headlight area. Have the auxiliary shaft flexible couplings inspected.
A clunking sound could also be caused by the main drive line U-Joints that bring power from the transmission to the rear differential. In any case these components are all under you vehicle any anything that causes a noise link this can be potentially serious so you are best getting a competent mechanic to inspect your Previa.