The linear theory of the generation of electrostatic bursts of noise by electrons trapped in chorus wave packets is developed for a finite temperature electron beam and a Maxwellian electron and ion background. The growth rate determined are qualitatively in good agreement with those obtained by previous authors from a more idealized model. Two connected instability mechanisms seem to be occurring: a beam plasma (electron-ion two-stream) instability commonly associated with intensification of the chorus power levels, and a transitional or bordering resistive medium instability commonly associated with chorus hooks. The physical reasons for the two mechanisms is discussed. In the second case, electron beams are difficult to identify in the particle data. An expression is obtained for the maximum growth rate in terms of the ratios of the beam and electron thermal velocities to the beam velocity and of the beam density to plasma density. It is anticipated that this may allow the observed peak in the electrstatic noise spectrum to be used as a diagnostic for the beam characteristics.