This paper reports the effects of sinusoidal microwave power Amplitude Modulation (AM) on the performance of Electron Cyclotron Resonance (ECR) ion sources. The study was conducted on the 14 GHz ECR ion source ECR2 at the University of Jyväskylä. The klystron output was intentionally altered by a variable frequency sinusoidal amplitude modulation. The average microwave power 350 W was modulated between 530 W and 180 W from 0.011-25 kHz. The integrated x-ray energy, the mass analyzed beam current and the forward and reflected microwave power were measured. The energy integrated x-ray signal responded strongly with low frequency modulation and was no longer observable at approximately 2.2 kHz where the signal strength became solely dependent on the time averaged power. The beam current responded in a similar way but exhibited a strong dependence with magnetic field. Qualitatively, we found source tuning parameters where AM effects were reduced also produced the highest currents of Ne8+ in Continuous Wave (CW) mode. Furthermore, these parameters are typically used for optimized beam injection into the K130 cyclotron. The dependence of beam current and the x-ray signal modulation on AM frequency for different magnetic fields are reported. A qualitative interpretation of the results will be given.