when i start up my truck, a pouring of a liquid sound comes from the rear of the truck underneath....you would swear it sounds like a huge leak is happening and pouring onto the ground, but i believe it is coming from the rear differential (whihc is not leaking).

after a few minutes, the noise goes away.

any ideas why it would be doing this?

when truck is started, i get a knocking sound coming from under the hood, prob the engine, is engine knocking caused by gas......after the truck is warmed up, the knocking is gone.

 

I am not sure about the water pouring question but the knock question you might want to run down through the sensors and see if they are working right and clean your MAFS, no knock is good and you can be killing your engine by leaving it

 

the nocking has gotten better, but the pouring wuestion sounds like it is coming from the exhaust.....right about below where you put gas in

 

I don't know what the problem is but here are a couple of random ideas, FWIW.

The two main products of combustion from your engine are carbon dioxide and water. It is possible for the lower part of the muffler to completely fill with condensed water over time. Stock Ford mufflers usually have a small drain hole to prevent this from happening but many aftermarket replacement mufflers do not. You can watch the exhaust and have someone rev it up to see if water comes out. A few drops doesn't mean anything but a spray might.

Your fuel injection system has a return line that constantly dumps excess fuel back to the tank when the engine is running. Normally, this is reasonably quiet but yours may be noisy for some reason. Or it could be something funny with the fuel pump itself.

 

like i said, after a few minutes, the noise goes away.