I've had that and narrowed it down to the stereo being slightly loose in it's fixings and the sound is travelling directly up. Makes sense as it's the closest place for the sound to come out of. Hope that helps mate.
Also you can prise that vent off quite easily to check to see if there's any parts in there rattling 
If something isn't bolted down securely in the zed it will rattle 
__________________
 <!-- Facebook Badge START --><a href="https://www.facebook.com/Johnwaynesboots" target="_TOP" style="font-family: "lucida grande",tahoma,verdana,arial,sans-serif; font-size: 11px; font-variant: normal; font-style: normal; font-weight: normal; color: #3B5998; text-decoration: none;" title="Kirk Sherratt">Kirk Sherratt</a><span style="font-family: "lucida grande",tahoma,verdana,arial,sans-serif; font-size: 11px; line-height: 16px; font-variant: normal; font-style: normal; font-weight: normal; color: #555555; text-decoration: none;"> | </span><a href="http://www.facebook.com/badges/" target="_TOP" style="font-family: "lucida grande",tahoma,verdana,arial,sans-serif; font-size: 11px; font-variant: normal; font-style: normal; font-weight: normal; color: #3B5998; text-decoration: none;" title="Make your own badge!">Create your badge</a><br/><a href="https://www.facebook.com/Johnwaynesboots" target="_TOP" title="Kirk Sherratt"><img src="https://badge.facebook.com/badge/1011934393.3054.324244859.png" style="border: 0px;" /></a><!-- Facebook Badge END -->
|