Its rather hard to describe but im gonna try, when i plug my headphones (Sennheiser HD407) into either my onboard audio or my Nuforce udac-2 i get the low frequency rumble, when audio i playing you dont really notice it, but if you turn down audio you can hear it, the rumble does not increase when you increase the audio either.
If i connect the headphones to my old media center with an old creative soundblaster soundcard the sound is not there, so im pretty sure its not the headphones.
Was told it could be due to a ground loop, no idea what that exactly means, but all the plugs in my house are 2 prong plugs hooked up to a power strip like this: Power Strip
Could an external soundcard maybe help or would that be the same as using my DAC? or should i maybe try connecting my DAC to a powered usb hub? Im running out of ideas to try :(
Forgot to add that for some reason the rumble sound seems to go away if i put my hand on the DAC or on my case when i have the headphones plugged into the onboard audio.