Either:

Loose connection (like tinsel lead)
Blown speaker (if you play loud)
Blown output in the H/U amp.

Give it's both at the same time, that's unlikely to be 1.

Take them out and have a look. Sounds terminal though.
 
i went and seen my brother in law tonight, he's an auto electrician. my tweeters are indeed knackered. need two new ones, he's going to put more powerfull ones in. no more bloody crackling then. :happy: