high pressure oil pump?

[Little Kelly Logger]

I would first like to thank anyone who might have an answer. I have a 03 F350 with a 7.3. Sometimes you can crank it up and it runs like it is brand new. Other times it makes a horrible racket and best I can tell it is coming from the front of the moter. It also misses when it is making that horrible noise. But when it deciedes to make that noise I can stop it by getting the rpms above 900 rpms. Please help I'm stuck.