[Roeland]

Just for the fun of it I thought on fitting a Police type of siren to my C5 (obviously not to be used on the public road). I sourced a small electromechanical one on eBay, but it is too quiet. Are there any recommendation out there?

[ace.cafe]

Alarm systems for the home or buildings have siren drivers and speakers which will operate on 12v battery power. You should be able to buy just the siren driver separately, and get a small horn speaker for it. You will need to make a switch circuit to activate it.
They can typically be programmed to output a wide selection of siren output sound types.

[Catbird]

...obviously not to be used on the public road...

That's no fun.   

[Roeland]

That's no fun.   

I should have put it between " "

[Craig McClure]

Alarm systems for the home or buildings have siren drivers and speakers which will operate on 12v battery power. You should be able to buy just the siren driver separately, and get a small horn speaker for it. You will need to make a switch circuit to activate it.
They can typically be programmed to output a wide selection of siren output sound types.

I will never forget the alarm system that screamed like James Brown "OWWWW"