Dell Inspiron 5000 series laptop - speakers make farting noises

Dell Inspiron 5000 series laptop (5502) - speakers make farting noises With increasing regularity, the audio from the laptop speakers will emit short bursts of a low frequency farting or 'brap' sound. The audio drivers are supposed to be up to date and the anomaly is never heard when listening via Bluetooth connected headphones. No Google searches I have made address this problem. I presume there is maybe some instability in the (class D (?) power amplifier that drives the speaker elements. I'm no stranger to component level SMT repair but I'd welcome comments before diving in.... Thanks