cobra alarm system

Hi all 
bit of a long shot here but, does anyone have a Cobra 5903 alarm fitted in their van?, probably in one of the older vans mine is 1994, just wondered if anyone had any wiring diagrams, manuals for that model, the problem I am having is that when the alarm is activated all is well for around an hour or so then there is an crackling noise coming  from the siren, turn it off wait a while turn it back on all quiet for an hour or so again  I am assuming it is some sort connection problem in the alarm module but without any diagram or manual a bit stuck, I have emailed Cobra, but no reply, any help appreciated
 Mike 
VWT4