MP1584 DC/DC step down converter cause whine in audio, how to avoid?
Most DCDC converters run well above 20KHz at your power levels .You are getting Audible noise and I think that you should put the oscilliscope.You will probably see ripple artifacts on the  5V    input and 1.5V output that are in the low KHZ range.When you scope the mosfet drain waveform you will probably see shimmering on the rising or falling edges depending on how you have got your scope triggering.Even when the DC input and load are constant the shimmering will persist.This low frequency crud cuts through the orthodox SMPS filter components like a knife through butter because those components are designed to attenuate the switching frequency.Experimentaly loading the converter may cure this because these low frequency instabilities tend to be worse at light load .If the load experiment nails this then you can increase the buck inducter and you will probably have silence or use a smaller buck converter which will in effect be more heavily loaded.If things get too bad consider linear post reg using a 1.5V   LDO and setting the buck up at say 2.5V .