D4 Balance shaft

[riggsy]

Currently have a 17plate XC60 D4 on lease. Since May 2017 to date I've only put 8000 miles on so far. Approx 1 week ago I noticed a high pitched hissing/buzzing sound both when on the move and also revving when stationary. Took car to dealership yesterday and left vehicle for the day so it could be checked, before leaving the technician revved vehicle and confirmed noise and said he would let car cool and do checks later in day.
Received call mid afternoon to say the technicians think noise is coming from balance shaft. Solution is to order new balance shaft but to complicate matters there is a new modified part due for release which they think might be available for December. I've been told the car will need to be booked in for 3 days for fitting of part.
Was told I could pick the car up and it should be ok to drive until December, something I am far from comfortable with. I am also concerned that if this is a major job to fit then other problems could be brought on in future if the job is not done to a high standard. To further complicate matters upon collecting vehicle dealership said they are unsure as to when they can order the modified part as it hasn't yet been assigned a part number on the ordering system. Can anybody advise on this 'Balance shaft' part and more importantly is the advice that I should be ok to drive around until December or beyond acceptable? I am far from happy with situation and will of course be raising my concerns with leasing company, I am just looking for some more information regarding this balance shaft component and potential problems that could arise in my current situation (i.e. engine damage if it fails) as I need to raise my concerns with lease company in more detail next week.Many thanks in advance.