1994 GMC Sierra High Speed Engine Knock

I recently changed the oil in my truck and then noticed while driving over 60Mph there is a knocking noise. It's not immediate when I go over 60 Mph it's only after I've been driving for about ten minutes. I only use the truck to go to work (15 minutes both ways on the highway). When I pull over to investigate the noise it only last about five minutes and then no noise. When I drive in the city there is no noise it's only at high speed when driving over ten minutes. I did notice when I do her the noise the power doesn't seem to be there if I give it more gas.