Check the oil level in your engine, and top it off. If it has been a while, go ahead and get an oil change.
Also, check your radiator fluid level, as well as your transmission fluid level.
If this sound is only present when you first start the van, it is probably just low on oil. If the engine hasn't been rebuilt, you may have oil leaking from your piston rings and onto your spark plugs electrodes, causing inefficient combustion.
Depending on how many miles this engine has, how hard it was driven, etc., it may be time for a rebuild if this problem gets worse. Main and rod bearings, valve springs, valves, etc. all wear out over time, and lead to louder than normal engine sounds. I’m happy to help further over the phone at
https://www.6ya.com/expert/jeremy_69f3cc28d95bf514