5 beeps playing every 30 seconds, whether key in engine running or key or out

[Wingtips92]

I have a 2004 plate Honda Accord 2.2 Diesel, which has randomly started playing a 5 beep tune every 30 seconds or so, it just beeps high-low-high-low-high whether the keys are in or out, whether the engine is running or off, the doors are locked/unlocked. I have a video too but I'm unsure how to attach so here's a link to it on Youtube. Has anyone experienced this before? There's nothing on the seats, the boot and doors are closed, the seatbelt tune still plays alongside it if I take mine off, fuel filler cap tight and closed.