This works best on black and white or generally monochromatic images without a lot of detail. The fake grain adds the “feel” of detail.
- Create a new layer by selecting Layer > New... > Layer. Change the blending mode to Overlay and check “Fill with 50% gray”.
- Select Filter > Noise > Add Noise. 15% is a good starting point.
- Blur the noise a bit by adding Gaussian blur via Filter > Blur > Gaussian Blur. Add 2.5 pixels blur.