Cutie, you don't mention wether the noise is from inbounds or departures. I guess since you live 40k out, its from inbounds in which case I think I have an answer for you. I fly the 320 and also live about 40k out, directly under the standard arrival routing into my base. I too have noticed this sound when sitting outside. It seems to coincide with deployment of speedbrake at high speed, say 320kt in order to make 250/10k restriction. Hope this helps.