Probably turbo seals. Get on the n54 motor section here. Those guys have seen stuff like this over and over and will be able to help you out.
http://www.e90post.com/forums/forumdisplay.php?f=58
If you do need to replace the turbos you can get them for about $700 from Tischer BMW or buy RBs turbos or Vargas is now making some pretty impressive turbos too.