Hello there!
I've just got a BB Bold and I noticed the same "creaking" noise at the bottom left of the back cover. It's very annoying.
I did almost the same thing as Thatzmister2u, but with a slightly smaller piece of thick paper.
I'm currently looking for the right single face adhesive tape that I'm planning to stick inside the backcover - This should do the trick
Cheers,
BB4L