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We present 17.5-MHz and 42.7-MHz low-noise Colpitts oscillators employing capacitive micromachined ultrasonic transducers (CMUTs), each composed of a thousand resonator cells electrically connected in parallel. The massive parallelism lowers the motional impedance, and thus, reduces frequency noise and provides better matching to low-noise oscillator topologies. The 42.7-MHz oscillator achieved a phase noise of -105 dBc/Hz and -148 dBc/Hz at offset frequencies of 1 kHz and 1 MHz, respectively in air. The performance is comparable to MEMS oscillators based on resonators with high Q in vacuum. The lowest Allan deviation of the oscillator was measured to be 4.7 times 10-9 implying a mass resolution of 0.96 attogram per membrane. In addition, using the 17.5-MHz CMUT resonator, the performance of the Colpitts topology is compared to that of the amplifier based oscillator topology.