2002 Chevy Blazer

I hear a rattling noise when in neutral but goes away when warmed up in "park". I can hear it when I drive but not as bad when I first start it. On straight away, and especially up hill, the rpm's will go when I press the gas but it will decelerate. It's like I'm revving it up in neutral because it will not accelerate. It shifts gears smoothly. Only code that came up was for the mass air flow sensor, that has been cleaned since. I've been told the fuel pump is going out. That was checked and the pressure is low.