Hi!

It is noise from different sources filtered by the PLL:s inside the signal generator. Most likly the noise at low frequencies, below 1k, is noise from the internal reference oscillator, probably an oven XO. The noise at high frequenies, above 100k, is from the VCO that is generating the output signal. Inbetween the a PLL is filtering the noise from those two sources in such a way it becomes flat.

This is a simplified explaination. The frequency generation inside a signal generator is much more complicated. It consist of multiple PLL:s to be able to generate an arbitary frequency with fine stepping over a wide frequency range, and do that quickly.