my M8 rear camera is also blurry and makes a 'buzzing' sound when trying to focus.. would you believe problem is resolved for me when i turn the phone upside down?? (also works in landscape when the front htc logo is on left, but not on right!!).. all this time and effort looking for a solution..
although it's not a permanent fix, it's a temporary solution that might work with others experiencing the same problem..
TM