Noise when powering headphones amplifier through USB port
Computer power supplies are typically noisy.  And that noise is getting into your headphones.  You need to add some sort of filtering to eliminate that noise before it gets into your amp.  Typically you would use some sort of LC low pass filter (LC = Inductor + capacitor).  In some cases you might need several LC filters in series.

I can't tell you exactly how to design this filter, since I don't know what frequencies the noise is at, or what the power consumption of your headphone amp is.  This is a good place to start.  I should also point out that a T or PI filter is very similar to just putting several LC filters in series.