Just finished replacing an engine in one today. It had a horrible knock from the top end at higher RPM's over 2,000. At idle, it was smooth. The problem turned out to be a bad lifter on #4 intake. With the valve cover off, you could see the valve float and the rocker clearance would get as high as .125".
The customer opted to get a junkyard engine instead of repair and guess what? This one knocks also, but not as bad.
Pull the valve cover off and check the rocker arm and is if any are loose or cracked also check the push rods. The 2.5 had a problem of putting push rods through the rocker arms. They would run good just make a hell of a racket up top.
Fireplug