About noise counterplan of LM3401

A capacitor across R1 will not lower the ripple as it is set with the hysteresis, it may filter some high frequency noise though. It will not hurt, but it may affect switching frequency and LED ripple current. A capacitor across D1 will filter significant noise, but it will also burn a lot of power. An RC snubber with a lower cap value might be a better approach. But using a diode with faster recovery characteristics and lower parasitic capacitance might do just as well. Putting a ferrite bead in series with the diode can reduce noise as well. In any case, it is best practice to have a solid ground plane beneath the circuit either on an inner layer or the bottom layer. That is one of the best noise reduction techniques. I only mention this because it looks like the 24V plane is all that is beneath the circuit.