Hello, I have a 05 pacifica with the 3.5. My gas mileage dropped from 20 mpg to 13 mpg. I changed spark plugs and got it to 14 mpg. I do have to replace water pump would that be the issue or something else?
Thank you

 

Could be any number of variables involved with a fuel mileage drop. Has your driving/start up pattern changed?..A lot of idling the motor will reduce overall fuel mileage.
Is the check engine light on?..means the computer is seeing something not operating correctly and will have a code stored in it to assist in diagnosing as to what is happening.
How does the car seem to operate? Engine running normally or hard starting/ performing poorly?
How many miles on the odometer?

 

 

In print..it's hard to tell what a "whine" is from...even from your twice stated concern for a water pump. BUT that would not drop you 6 miles per gallon.
The 3.5 is timing belt driven with the water pump thrown into the mix, at your age and mileage...has the belt ever been changed. If not you're running on "borrowed time" to a complete engine failure as the 3.5 is an interference type motor.
The stated info you have provided does not give enough to be able to suggest further than dirty injectors or intake air obstruction( the other 2 things needed for a running engine).
Good Luck!