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noise is everywhere. Oscillators especially are susceptible to noise, since their feedback mechanisms amplify the random noise over and over again until it gets very large. Synthesized sources are susceptable too since they typically take a low frequency clock source, and use various tricks to multiply that low frequency up to microwave frequencies--multiplying the noise in the process.
source A is -60dBm below carrier and source B is -70dBm below carrier. So B source is having lower phase noise.
The sources of phase noise are 1/f noise( due to crystal cut etc), flicker noise(due to semiconductor material) and thermal noise( due to thermal agitation of carriers in resistor).