As title says, WIN7, game via steam so should be up to date. ???

Do you have any mods? That's a strange issue you've got.

Nope, no mods, just started playing, been playing for abour 5-6 hours... and yep, its pretty strange and annoying...

Kind of grasping at straws -

I have found FO3 and FONV to be codec sensative. Using radaway causes some audio effects, it might be a codec issue. Try Nirsoft's InstalledCodecs utility, This allows you to disable/enable codecs without uninstalling/reinstalling them: try disabling all codecs EXCEPT those from Microsoft, , Fraunhofer, and Spiro. And the cool thing is - you just reenable them if it makes no difference or you want to find the culprit.

If that doesn't work post your dxdiag.