Smoothing and decoupling power source input
The noise you're hearing is most likely coming from the switching frequency of the power supply; perhaps mixed down by the radio's Superhet receiver.

In situations like this, where you want a low noise output from a switching power supply, extra capacitors are not your answer (although they help). The problem is that the ear has a very high dynamic range to audio. So even if you filter it with decoupling capacitors it may still be audible.

The answer is to use a linear regulator such as an LDO to step the 18V from the switcher down to a lower Voltage (say 16V). This step down "slices off" the hum sitting on top of the 18V DC.

Here's an example of an LDO from LTC with a 36V input range and adjustable output up to 36V.