I recently started noticing some quite loud crackling noises during the last part of the initial heating process after tuning on my Breville Dual Boiler.

You can have a listen in this video: http://www.vimeo.com/615057400

It starts after reaching about 70 degrees celsius and continues until it has reached brewing temperature.
Could it be an indication of limescale on the heating element in one of the boilers? I know there is a descale prompt on the screen - but I have always used bottled water with proper mineral content (maybe a TAD too hard though) and I regularly drain both boilers to get rid of high TDS water accumulating.

Any advice and input is highly appreciated!

it is scale in the boiler

Really? How does scale make that sound?

Take the old kettle, remove the lid and start heating.
water penetrates into the scale and turns into steam, that is, vaporization occurs in the scale itself, hence the noise (steam flow under high pressure)