any noise from an engine is never a good sign, however crackling is a poor diagnosis, unless we are talking rice krispies
Best bet would be check oil and water levels firstly, but to be honest a strange noise could be a loose drive belt, a loose pulley, or even a bearing on the way out in a pulley, maybe something like the water pump or the tensioner on the timing belt,
timing belt would be a serious issue so best get it checked out straight away,
Surely if it was you'd have the snap and pop as well Sorry couldn't resist.
I don't think so because if it was the fan belt or timing chain , they would have heard them snapping