[moltuae]

I had a synthesised 'horn' that I fitted to one of my first cars, back in the late 80's. It was in addition to the regular horn and really just for novelty value and comic effect but it was quite loud. It comprised of a sound generator (with about 8 different buttons/sounds), an audio amp and a weatherproof speaker under the bonnet. It had air horn sounds and sirens but I think the phaser sound was what I (and my college mate) used most.

I've made similar sound generators over the years for various things, most recently a steam train whistle for one of my customer's garden centre displays. It's quite inexpensive to knock a synthesised sound generator together, especially these days with the all the readily available electronic modules from China. You just need to find a generator module that makes the sound you want (or you can use an 'mp3 module' that will play a recorded sound), an amplifier and a speaker.

A horn speaker would probably work best for a car, such as these for example:
https://www.ebay.co.uk/itm/White-Ext...o/350834652750
https://www.ebay.co.uk/itm/Eagle-15W...r/232578273146

Cheap mono amp modules are easy to come by. Something like this should work well:
https://www.ebay.co.uk/itm/TDA2030A-...d/292583332964

Then you just need to find the right sound(s). How about a fog horn?
https://www.ebay.co.uk/itm/Nebelhorn...r/302381766306


Edit: I could let you have an mp3 player module if you need one Ray, complete with this steam train whistle if you like
https://www.dropbox.com/s/ndt2b0aczza13hp/1.mp3?dl=0