2001 Other Jeep Models Engine Rattling

I have a 2001 Grand Cherokee Limited V8 with 159000 miles there is a rattling on my engine when i start it up then it goes off. when i rev up the engine to 3000 rpm the engine starts to rattle. a friend of mine told me it might be that i put bad quality of gas so he recommend me to put a fuel system cleaner but didnt work. another friend told me it was the oil pump since i was losing oil pressure. the oil i used is 10w30 high milage but my friend told to use 5w30. im confused right now im afraid to use my truck cause i might break down the engine/