1. Create a new layer, and fill it with median gray (shift-backspace).
2. Change the blending mode to overlay. It becomes invisible.
3. Doubleclick on the right side of that layer, and a Layer Style dialog box pops up.
4. Check the Pattern Overlay box, and again set the blend mode (within this box) to Overlay blend mode.
5. Now, click the down arrow on the pattern box to select your pattern. Look for patterns that are close to median gray.
6. (while in step 5) If you don't see a pattern you like, click the right pointing triangle and load some new patterns.