Just received my new 13" 2011 MBP. I immediately removed the stock 320 and installed a Momentus XT. Once powering it up, I was hearing these faint clacking noises coming from the left side. It doesn't do it all the time but when it does, it's really annoying, especially in a quiet room. I chalked it up to a poorly designed or bad Hybrid drive. I removed the drive and installed an Intel X25-M, exact same thing. I should've known it wasn't the hard drive since it's on the right side. I guess at this point I might as well send it back. I'd really like to figure it so I don't have to mess with a return. I've done some searching and it seems it's a fairly common problem with MBP's. I'm going to post a link from You-Tube with a guy that has a 2009 model but it's in the exact same area and the exact same noise.
http://www.youtube.com/watch?v=K_2QiE3EN4M&feature=related
http://www.youtube.com/watch?v=K_2QiE3EN4M&feature=related