What is making this noise (Ultrasonic sensor output)?
It appears that this 100 Hz noise is the lowest frequency.


100 Hz will be the ripple frequency from the output of a bridge rectifier used in a power supply that takes 50 Hz AC as its input source. Below is a picture of 120 Hz ripple when the input supply is 60 Hz: -



Here's what half wave and full-wave (bridge) signals look like when aligned to the input AC waveform: -



Half wave tends to produce a ripple at the same frequency as the incoming AC but full-wave (bridge) produces double the frequency because both halves of the AC waveform are combined in the rectification process to make DC.