"Thunder heard!": rumbles, crackles & bangs - why the difference?
The noise we hear when a thunderstorm is in progress is caused by the near-instantaneous expansion of air (due to intense heating) along the path of the lightning discharge. The type of noise we hear (rumble, crackle or bang) depends upon the distance of the observer from the lightning path and the path 'structure', e.g. simple stroke, branched pattern etc.
A lengthy drawn-out rumble/crackle of thunder is due to the slow speed of sound (330 m/s) through air - the lightning 'flash' of course travels at the speed of light (several orders of magnitude faster) - effectively our eyes see the discharge as soon as it happens; the sound wave takes much longer to reach our ears, unless the lightning strike is very close by - when we would experience a violent 'bang' . . and hopefully nothing worse!
When lighting undergoes much 'branching', both within and outside the cloud, although the whole flash would take place over a small fraction of a second, the sound waves from each part of the path (and the branches), take different periods of time to reach an observer: the sound will be heard as either a continuous crackling (relatively close-by storm), or low-intensity 'rumble' (distant storm), or a mix of the two, again depending upon the character of the discharge path.
The reason why near thunder 'cracks' and distant thunder 'rumbles' is this: sharp cracks are composed of sound waves with high frequency (or short wavelengths); these are rapidly damped owing to irregularities in wind & temperature. The lower frequency (longer wavelength) portion of the sound wave is not absorbed anything like as much, and therefore it is these we hear from distant storms.
Thunder, under normal atmospheric conditions, has an absolute maximum 'travel' of about 20 km, and more usually 8 to 10km. In mountainous areas and under conditions of 'anomalous' low-level refraction, then greater distances may be achieved. (See also here)