Sounds annoying but sometimes a higher frequency oscillator could have lower phase noise than a lower frequency oscillator, even they use the same active component.
Leeson equation stated that: doubling the operating frequency the phase noise increase by 6dB, but in the same time, doubling the loaded-Q of the resonator the phase noise decrease also by 6dB.
Phase noise improves with higher loaded-Q of the resonator, and because the Q factor of some reactive components increase with frequency, it is possible to get better phase noise at higher frequencies.
Usually can get lower oscillator phase noise at higher frequencies when the output signal is extracted from the resonator, in this way using the resonator response selectivity to filter the wideband noise spectrum.