After 5 days of hopeless troubleshooting (severe crackling coming both from the laptop speakers as well as headphones was driving me crazy) and several calls with Dell support (the best they could offer was motherboard replacement), what finally solved the problem for me was reducing the DPC latency.
Specifically, based on recommendations from the community
1/ I installed free DPC Latency Checker, which showed my DPC latency being constantly at around 16000us with acceptable levels being below 2000us
2/ I tweaked the Power & Sleep inside Win10 - various changes offered various levels of improvement, with the simplest advice being 'select any power plan other than the Balanced one' and ‘remove sleep / power saving options’. That substantially helped to get rid of the crackling, but the latency was still at around 2000-4000us
3/ after installation of the Dell Power Manager utility and selecting 'Ultra Performance' Thermal management, the latency finally reduced down to just 1000us, while - ironically - the notebook became significantly quieter than before