My truck sounded like a monster, now it sounds like a chicken help please?

Ok so here is the problem.  One day it was loud and drove great.  The next you can't even hear it coming down the road.  When I drive it the throttle response sucks when I try to floor it to pick up speed.  All you can hear is a loud hissing sound and it doesn't go anywhere.  So basically the engine seems to have lost some horsepower cause it won't go anywhere fast and I can only drive it on the main road.  I thought it was a vacuum leak but I checked all my air lines but I couldn't find a leak.  Transmission shifts good and level is good.  Before I started replacing parts it bogged out at 3500 rpms now its closer to 4500rpms.  It isn't throwing any codes so I'm at a loss.  Everything that I have done have helped the engine run better but hasn't remedied my problem.  What I've done; Replaced spark plugs and wires, throttle position sensor, idle speed sensor.  Any ideas? thanks for you time.