First thing to do is check the suspension for any loose/worn out parts, also
check the tires for any signs of problems. Then you may want to rotate them
from front to back to see if the problem goes away.
If you can rock the van from side to side from the outside and hear the
noise it is the sway bar bushings, a common problem for this especially when
the temp gets colder. But without hearing the noise myself would be hard to
tell what it is.
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