uneven floorboard under carpet

We have a 10 year old 2-story house. We bought it about 3 years back. We have uneven flooboards under the carpet in the upper story. It is mainly in the hallway area. There was squeaking here and there for some time. Latley it seems to have become worse. In one corner next to a door frame, the floorboard has become noticeably uneven. Is it something we should be concerned about? I dont see any obvious water damage anywhere. I have not lifted the carpet yet to see what is happening under it.
There are also some hairline cracks in the drywalls. In one place it is diagonally from the upper corner of a doorframe. The previous owners also had the cracks and told us that they patched it up. At that time we wrote it off as cosmetic cracks due to heating/cooling. Can this have anything to do with the loose floorboards?
Any comments/suggestions are welcome.
Thanks. -Dipu
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