Trooper electric roof

You have a very rare variant of the Trooper, so you may have to wait a while to get an answer here! Most Troopers were built with A/S's own solid-sided roof, and relatively few were built with the end-lifting fabric roof (originally made, IIRC, by Reimo) and of those, relatively few had the electric motorised option.

We had a good look around one a few years ago, but decided to go with the solid roof instead. We didn't have a chance to start the engine (because it was on a dealer's forecourt) so have no knowledge of any siren. It's quite possible that there is a warning sounder if you start the engine with the roof up, but I doubt it would need to be incredibly loud, as you report. So it seems to me more likely that you triggered an anti-theft alarm. Unfortunately I don't think that any "standard" alarm was fitted from the factory (our 2005 model doesn't have one) so if that is the issue there probably won't be a "standard" answer.

Incidentally, if you're not already aware, you need to watch like a hawk while that roof is coming down, to make sure that a gust of wind doesn't cause the canvas to catch in the hinges. The example we looked at had extensive (poorly repaired) damage to the canvas where it had been chopped through on the hinges!

Not sure why it needs to be incredibly loud - but you may be glad of it someday. I've driven off site with the folding roof not latched down on a previous A/S conversion, and I only knew it was lifting under air pressure when I braked and it came down with a bang! Fortunately no damage done as it was fairly low speed - could have been very different at motorway speeds...