Yeah that's what I thought. Yes I hear it even with no audio signal.
On 1.0x gain, I hear nothing, which is what I expect. On the 2.5x gain, I get that kind of background hiss that one would hear with a portable media player and IEM's. It's subtle, but I can definitely tell the difference between the 1.0x and the 2.5x gains.
I don't think it was my desoldering job...the circuit looked pretty clean when I took out the resistors.
This happens on multiple computers. And actually, with the O2/ODAC, I discovered that one of my USB ports on my MacBook is dying because the audio produced from that particular port is all distorted like one would find in a horror movie. Switching USB ports fixes this issue. XD
When using 1.0x, you are not amplifying anything... That's the reason you do not hear any sort of unwanted artifacts.
I have tried to experiment and also noticed a very low level hiss when using too strong gain with sensitive phones. Like 2.5x with headphones that needs 1x in reality... You should use the lowest gain needed from several reasons mentioned on the author's blog, one being audible distortion with non-ideal gain.