The English word van was borrowed into Welsh as
fan. This wouldn't normally cause too much of a problem;
man is most commonly treated as a masculine noun, and so
wouldn't undergo soft mutation to become fan.
However, like many Welsh nouns, man has traditionally
been treated as either masculine or feminine in different areas of the
country. The feminine form survives particularly often in idioms like
yn y man / yn y fan
(in a while). As a result, yn y fan
could mean in a while or in the
van! Context will help you to decide which meaning is correct.