[GTP Dad]

You don't say how many miles you have on the van but this sounds like a worn out strut. They will make a knocking sound when they begin to wear out. If you remove the front tires and check to make sure the sway bar is not broken or loose that will eliminate one source. If it proves to be ok then the struts will need replaced.

You do not need to go to a dealer to have them changed most shops who do suspension like Firestone, Midas, Sears, Pep Boys, Monroe and etc can all do struts for you. You can do the job yourself but you must have a strut compresser designed specifically for struts. Anything else is dangerous. If you are not familiar with the job have someone else do it for you. You will also need an alignment after the struts are changed.