I have a QC25 purchased in BIC Japan in April 2015, the right speaker failed in August 2015 and I had it replaced under warranty by Atlas Singapore (local Bose distributor).
The left speaker started having intermittent problems in September 2016, warranty was already over by this time. Nevertheless after reading on other forum about this being a manufacturing defect, I went back to Atlas Singapore, in October 2016, and they replaced this as a courtesy with a 2016 manufactured unit. The support personnel I spoke to was very helpful, and she confirmed that BOSE has acknowledged there were manufacturing issues with 2014 and 2015 units.
So if you have a problem, get it replaced and I would not recommend trying a DIY fix.