Rattle at start up?

[troyjuly]

I have searched these forums for my problem and from the sounds of it I may be looking at a water pump but would like a little more input.
At start up of my 97 boxster with 70000 miles on it I hear a slight rattling sound from the engine compartment. This is quite noticable when the engine is cold. If I rev the engine slightly the sound stops as well as it is not as noticable when the engine is warm at start up. It is not noticable at all while driving.
The only other thing I have noticed is that I have to add about a liter of coolant every 2500 miles or so but see no leaks anywhere. Even on the floor of my garage there are no indications of anything leaking in the car.
Any input would be greatly appreciated as well as if it is the water pump any detailed links to changing it out myself would be great.
Thank You.