This seems to be an issue on the 15''. From what I understand, this is a design issue. The speakers will make a rattling sound on particularly low quality sounds or compressed audio files. You shouldn't get rattling/vibrating on high quality sound such as 320k mp3's, audio cd's, audio from commercial dvd's etc. unless you turn the volume way up. The speakers tend to rattle when the volume is past 50%. I've tried this out on 6 different mbp's, and they all did this to a certain extent. Most people aren't as sensitive to it. So right after I post you'll see people saying that their macbook pro is the most amazing thing ever, that it's quieter than a vacuum sealed chamber, and that the speakers sound like a symphony in their living room. I'm sure some people have very minimal vibration, but because the crossover on the right side speaker/subwoofer combo seems to be set so high, the right speaker tends to vibrate and not sufficiently reproduce particular frequencies. You can also tell that apple could give a sh*t about sound reproduction because the speakers are not lined up on the left and right. Most people hear the sound as coming out strong on the right side because for some reason the subwoofer can be localized on these machines (normally you can't localize low frequency sound as easily). So poorly designed speakers + poor alignment=problemos.