Shining ALP dark radiation

String scenarios typically not only predict axion-like particles (ALPs) but also significant amounts of ALP dark radiation originating from the decay inflaton or a more general modulus. In this paper, we study such non-thermally produced relativistic (but massive) ALPs to photons. If are sufficiently highly energetic, contribute $\Delta N_{\rm eff} \gtrsim {\cal O}(0.001)$ and have mass $m_a\gtrsim$ MeV find that, using observations X-, $\gamma$-rays, CMB BBN, very small values ALP-photon coupling can be probed, corresponding an origin at string (or even Planck) scale.