hi, and welcome to BimmerFest!
To be honest, I've never heard of an airbag that squeals.
We need you to give us a bit more info.
Does it only squeal at start up?
Does is still squeal at a warm or hot start up?
Can you pinpoint more closely where in the back the squeal is coming from?
Are you sure it's not coming from the engine bay?
2000 740iS. MKIV, 16x9, DDEs controlled by Euro fog light switch, reversed steptronic +/- pattern, paddle shifters, clear corners, Basslink, iPod video with stereo audio, TV tuner, gauge rings, ///M pedals, backup camera with 'on demand' switch, quad brake lights, Dinan engine & tranny software, windows/sunroof close via remote.
2003 540iS. MKIV, 16x9, PDC, CWP, TV tuner, backup camera with 'on demand' switch, paddle shifters, windows/sunroof close via remote, iPod video & audio.