I gave it a shot and tried two approaches:
Left: Particles, Right: Shader.
Here’s the file, if you’re interested: pneumatic_silencer.blend (1.2 MB)
The particles are just a low poly sphere with a basic metal shader that is distributed through the volume of the object. Unfortunately it seems to take quite a huge amount of particles (50000 is quite large for my laptop, at least) to eliminate obvious holes - even though I used a solidify modifier to concentrate the particles on the perimeter of the object.
The shader is based on the voronoi texture and RGB-Curves to create a ‘bubbly’ bump map. To eliminate large gaps between the ‘bubbles’ I had to layer this setup on top of itself quite a few times, which is why put it into a node group. It makes the setup pretty slow as well.
In case you’re using cycles you could also modify this to use the bump map to drive the displacement. it would require quite the dense mesh though.
There is definitely a lot of room for improvement, but I hope this gives you some ideas 