Cause and effect ?
I'd suggest not to use a noise DCDC in this case,

or if you do , you need to filter its output well, may be using a low noise LDO,

Hello
Explain more about LDO.

 

Well , what do you know ?

two basic types of power regulators,

those that are collectively called DCDC, use a pulse of current to charge an output circuit.
advantage, they convert power, so apart from minor loss power in = power out
disadvantage , that pulse, cause a lot of noise, that can cause problems
you have this example.

Those that are called collectively regulators,
example being the LM317
These act as a variable resistance between the load and source,
such that the output voltage is kept "constant" no matter how the input voltage or current taken changes. ( within limits )
advantage, lower noise than a DCDC, less filtering needed, dead cheap
disadvantage , they absorb power, W=VI, as they current goes up, or the voltage drop across the regulator increase, the power lost in the regulator increases.

originally regulators, would drop out when the voltage across them was to low. like below a few volts.
but about 30 years ago, there was a change, to regulators that only needed a much lower voltage across them
Low Drop Out ( LDO ) ,the name LDO is now collective, for all of this regulator type.

 

(some text removed for clarity)
I need frequencies less than 2.2 MHz.
What filter do you suggest?

The characteristics of the filter depend heavily on what kind of signal (the spectrum of the signal) that you want to get through. You have not yet mentioned what signals you want to receive.

 

Look at the output of your DCDC ,
and see what noise you have there,
remember to keep the scope earth very short and directly connected to the dcdc output , don't use that long wire thing on the side of the probe,

 

Apparently a large part of the noise on the amplifier output is capacitively coupled to the amplifier (through the air) from the circuitry of the DC to DC converter.

 

The characteristics of the filter depend heavily on what kind of signal (the spectrum of the signal) that you want to get through. You have not yet mentioned what signals you want to receive.

Hello
I need frequencies below 2 MHz

 

The noisy AM broadcast band is below 2MHz. "A" for amplitude and an AM radio picks up lots of amplitude fluctuating noises.

 

Hello
I need frequencies below 2 MHz

Since 2 MHz is very close to the frequency of your noise source, you will probably have an easier time just lowering the amount of noise coupling into the amplifier rather than trying to filter it out. So, I would concentrate on layout and decoupling for now.

 

The schematic shows capacitor pC. Can it actually run without using value like pF ? value is so small, did i miss final regulator ?

The amplifier might benefit by being isolated and or shielded more so for this application.
recommended groundplane and layout on datasheet and recommended capacitor X5R
The dc converter is low noise however alot of great parts but triple amplifier very sensitive
https://www.analog.com/media/en/technical-documentation/data-sheets/1931fa.pdf