First things first... are you sure that it has an alarm siren and that it had been chirping before? If it has and you know that it had been chirping
before then you have a defective siren unit. If not, then check if you have a siren by trying to activate it (try to steal your car). And even if you have
a siren, it does not mean that it has to chirp (like most aftermarket burglar alarms) when being armed. In some cases, the way the alarm responds
when it is activated is set thru the Car & Key Memory (you need a BMW dealer's equipment/scanner to do this).