I've just experienced the same symptoms with my QC35 II; crackling noise with ANC on left speaker, and it intermittently not working at all. Lightly "hitting it" seemed to resolve it for a while, further suggesting a glitchy/loose electrical connection, until it fully stopped working. Disassembling it, and probing it, all connections seemed to have signal and be well, but I accidentally poked the speaker membrane, which made it sing! So a "speaker driver replacement" should probably do it. I temporarily added a piece of rolled tape to press down on the speaker membrane at the connection point to make do for now (the speaker driver is behind a glued cover, requiring a hot air gun and glue to restore properly). See How to repair Bose QC25 or QC35 - speaker replacement DIY or Replacing Blown Speaker on Bose QC35 I & Bose QC 35 II.