Low rumble from onboard audio and DAC

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.

It would help to have some sort of picture, or not.

The ground loop is basically what grounds your electrical system to the ground, and it generally pulls a majority of static away your electronics. I do believe that this is the issue, there is a static build up in your system that is either gradual or sustained, and your DAC/mobo gets grounded when you touch it. Which causes it to go away. The solution to this would be to ground your system, also known as a third prong in your plug, though I have no clue how it works where you live. The other solution would be to ground it using an external thing, like a grounding bracelet wrapped to something metal touching the ground.