Hiss on headphones - HP EliteBook 840

Hi all - I hope this is the right place to seek a bit of help, feel free to move if not.

I've just been given an Elitebook 840 by work, and installed Windows 8.1 on it; everything is fine, but when I have headphones plugged into it, there is a faint audio hiss whenever sound is played, which does not immediately disappear.

New email? "Bing", plus "ffffffffffffff" for about 8 seconds.
Move audio slider? "Bong", plus "ffffffffffffff" for about 8 seconds.

Playing music via Youtube / Soundcloud produces the same audio fuzz in the background. There is a DTS control panel, but changing anything on this doesn't seem to make any difference. The fuzz is the same volume, regardless of the audio volume. I've tested with different headphones and it appears to be always there cheapish Sony on-ear ones, AKG K450's.

Any ideas of how to get rid of it? It's driving me insane!!!

strange that its with headphones, if it was from the speakers i'd have guessed feedback. Could be worth trying disabling the mic anyway just as a test.

Try re-seating the hard drive making sure not to over tighten the screws.

I've had a few of the i7 versions with the AMD gfx play up in a simular way. Had to send some back for multiple repairs and they come back saying "replaced plastic parts".

Hopefully you didn't wipe out the restore partition when installing 8.
So factory restore (IIRC F11 at boot), update BIOS, latest drivers all from HP website, still a problem, it is probably still under warranty get HP to fix it.

It seems to be low impedance headphones that are causing (or illustrating) the issue. With my AKG451's, the hiss is there and they are fairly loud with volume sliders low; I noticed that my Creative Fatal1ty headset was much quieter when used with an iPod or phone, so I tried those with the laptop and the hiss is not there, though I have to have the sliders up more.