Eduk,
The SA44B works by capturing 200 kHz patches of spectrum and processing them. There seems to be some 200 kHz periodicity. I noticed that the peak-to-peak periodicity improves for the stronger signal, so it looks like what you are seeing is the IF filter shape on the SA44B’s noise level.
Because your RBW is 100 kHz, the software has a hard time correcting for IF flatness. If you reduce your resolution bandwidth to 30 kHz, the software will have an easier time correcting the IF flatness, and this ripple should be reduced.