We have modelled the effect of two intrinsic noise sources on the flux noise spectral density of (Double) Relaxation Oscillation SQUIDs ((D)ROSs) based on hysteretic Josephson tunnel junctions. An important noise source is the spread in the critical current of the SQUID due to thermal fluctuations. Critical current noise mainly determines the noise on the average output voltage of DROSs with high flux to voltage transfer. A second noise source is the spread in the relaxation frequency due to the random interaction between the Josephson oscillations and the relaxation oscillations during switching to the zero-voltage state. This effect can dominate the voltage noise of a ROS.