evil microphone feed back noise in right ear

Hi. ive got a pair of QC 35’s that were working perfectly but now have a persistent squealing noise whenever the noise cancelling mode is engaged. Bose have so kindly offered to sell me a new pair at a discounted price but its still $300 i dont want to have to spend after only 16 months use. ive got the latest software update, the state of charge of the battery makes no difference or whether im using them wired or wirelessly