Constant beep noise from engine bay

Good afternoon. I was out changing some headlight bulbs. I turned the ignition on but didn't start the engine. I noticed a constant beep coming from the engine bay near the radiator which lasts about 20 - 30 seconds then clicks off. Sounds like a quiet version of the warning beep in the cabin but that isn't where it was coming from as it kept going with the key out and doors shut and the car locked. Can anyone tell me what it is. Is it a relay, glow plug something else?? I'm totally foxed

Thanks for the replies. That's what I get for going under the bonnet in the first place. I know plenty about problems with buildings but knowt about motors. Always willing to learn though! Cheers.