Something I've noticed is that I can hear a high pitched noise from my Crucial M4 during heavy write operations such as extracting a 2GB zip file.
The behavior is not unique to my Thunderbolt setup, but I suspect its exposed open-air nature makes it more apparent. I heard the same thing when I opened the case of my PC (which housed a 128GB Crucial M4) and when briefly using a bare USB 2.0 to SATA adapter on my notebook (connected to another 256GB Crucial M4).
It's not something I am worried about, but my friend warned me noises usually mean dying capacitors or a bad power source. I think I just have sensitive hearing, combined with how silent my iMac is when the hard drive is asleep.
For the curious, the sound is a much
quieter form of this: https://www.youtube.com/watch?v=uuijfJfQxts