Got an issue with the alarm on my 850.

The central locking works using the fob and indicators flash as normal. Upon turning the ignition on the alarm siren will sound but intermittently (like a contact is loose and making an intermittent connection). Will also continue doing this when the engine is started. The siren doesn't sound when ignition off or on stage 1.

The battery is low and I've changed it for another one which is a bit low but I've had low batteries before and the alarm has never done this.

Any ideas?