With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.
In this circuit it's not easy to apply superposition, and analyze bias and signal independently. Other way you can think it would be the 15Ω resistor as part of the collector resistor and picking frome where the noise is smaller, or summing cancelling out the noise using an inverted signal.
The point is the noise rejection capability is given from the DC and the ratio of those resistors, not from the signal itself. Then it's a matter of tweaking the gain for best cancellation and bias point for best efficency, depending on the shape of the ripple.
JS