fuel pump won't prime

in the mornings or after a period of time
when I turn the key to the on position fuel
pump will not prime unless key is left in on
position for 5-25 minutes. when car has sat
in a parking lot of in the sun the fuel pump
will usually prime either right away or after
just a minute. & sometimes before it will
prime if I turn key off then back on I will
hear a knocking noise from the fuel pump
but until it will eventually prime itself.