Oh i service it myslef of course, never waste money on professionals, i do all my own electrics, building, plumbing car repairs, etc etc

mind you my house fell down last week and i have no-one to sue

Yes I do service my own alarm (god knows why I own an alarm company I could at least get a free service) twice a year!

That is true, but another way to look at it is this, if he gets one of his engineers to come out to it (not telling them it's his) and lets them service it, he can evaluate how well they know their stuff (random error time)

Back to the subject at hand:

Doug, I do hope you get it sorted, just thinking it could be a stray wire fragment / loose connection to the Speaker connection in the keypad it's possible it's connecting making the sound then disconnecting - weird to even suggest but well worth a look after checking everything else that's been suggested.

hope you get it sorted