Possible source of a 120Hz noise on ADC?
(This is mainly expanding on Nick Alexeev's comment — I can't say whether there are other effects to consider.)

You will see this unless you shield your photodiode from all ambient light or you take it outdoors, because almost any line-powered electric light source will flicker at 120 Hz insofar as it flickers at all.

The flicker is 120 Hz rather than 60 Hz for the same reason the output of a bridge rectifier has a frequency of 120 Hz: the processes along the way from the AC waveform to the emitted light don't care about the sign of the current.

The exception to this would be if you used a low-quality LED lamp that connected a single series string of LEDs across the line — this would effectively be a half-wave rectifier and therefore flicker at 60 Hz. But good LED lamps will have a full-wave rectifier, antiparallel LEDs, or a more elaborate power supply.

Incandescent lamps are electrically resistors so the heat and therefore the light varies with the square of the current, and the square of a sine has twice the frequency (but of course, the thermal mass acts as a filter, so you can't usually notice the flicker).

In fluorescent lamps, the behavior depends on the type of ballast in use. Old-fashioned “magnetic” ballasts pass line AC to the bulb and therefore the arc will flicker at 120 Hz (since, again, it lights up the same no matter which direction the current is flowing) while electronic ballasts synthesize higher frequencies, much like switch-mode power supplies (but still might show 120 Hz variations since they are still influenced by the line voltage).