I have had my Sprint Hero for about a month now, and the creaky back button is about the same, maybe a little better. It's not the kind of thing you get used to. I will copy below what an HTC tech wrote to me regarding the creaky noise. Looking back, I wish I would have simply gone into a Sprint store and exchanged it since I had just received it. As I mentioned, I visited a few different Sprint stores and got different reactions. You may want to go to a different store and attempt to swap it out.
From HTC:
I understand how important it is for your device to feel just right. Like most electronic devices, not all devices feel the same. Some have a tighter feel; others may have the creak you have described. It is not normal to the point that every device has this sound, but I do not believe it to be that abnormal. Not being able to actually experience the creak that you are experiencing makes it difficult for me to judge. However, I do feel that if you have misgivings about the way the device feels, they should be addressed. If you have just received it, Sprint should be able to swap it out for you if it does not feel right. If they cannot do so, we do have a repair department that would be able to assist you. However, I would not send it in unless it were entirely necessary, as it may be much ado about nothing. If the button does not function properly or seems to deteriorate, you may need to seek repair. Otherwise, the device should be fine.
