Knocking sound in OverHead when accelerating.

Hello, I went in for a tune up on my Volvo 880 with d16 motor and the tune up could not be performed because one headvalve appeared to be pressed up way too high and had absalutey no free play. Even when the adjustable bolt to control free play was skewed out to the max.

What could the reason be for this? And what do you guys think has caused the headvalve to eliminate it's free play?

The reason I went in for a tune up was because I started to hear a knocking sound in the motor when accelerating.

Recently I have changed my turbo about 8,000 miles ago.

Thank you.