Rolling thunder.

[Rob A]

Normally a thunderclap is just that. Single event, percussion like a firecracker then sounds, rumbling, continuing for a few seconds, tens of seconds.
Weve just had a thunder clap the distinct initial percussion crack, that went on for twenty seconds or more, followed by rolling continuous rumbling going on for another minute.
It came maybe ten seconds after the sky light from the lighting.
Really odd. What causes it?
Do I need to join a met forum?

[eggs]

This sort of makes sense:
http://www.askamathematician.com/2011/09/q-why-does-lightning-flash-but-thunder-rolls/

ie - the sound is being generated along the length of the lightning strike. Its sharpness and length is dependent on how it lies relative to the listener.
The long rolls I hear are generally associated with sheet lightning that runs cloud to cloud. ie a long way lengthwise away from me.

Another phenomenon is mentioned here:
http://wattsupwiththat.com/2011/04/17/atmospheric-audio-ducting/