Hello and welcome.
Does the noise speed up as engine RPM goes up, or does the noise seem to be related to vehicle speed and not the engine speed?
Does the engine run smoothly?
OK. Is it a constant noise? Like knock-knock-knock...
Does the noise change if you are driving and then turn the steering wheel?
OK. Then this does not sound like an engine noise. If it were, it should do it when revving in park, or especially in drive, since that puts a heavy load on the engine. This sounds like suspension problem.
A loose tie rod can give a noise like that, as well as worn out lower control arm bushing. These are pretty common. The faster the van goes, the faster the wheels spin, the more pronounced a loose component will be. I suggest having the front steering and suspension checked for loose components, especially those i just mentioned.