hmm thanks so far for the info.
Seems to be an isolated problem with my headset, maybe even a small batch. Not sure at the moment, Bose are looking into it and the headset is going back again for testing.
Tried two others this week owned by other guys I work with and theirs seem to be perfect. They were also 3-5 years old. Its possible something has been changed recently that is causing an issue.....
Hopefully get to the bottom of it.