Sagging Floors

[Chris1981]

I have a townhouse that I bought new in 2006, it is 5 years old. I have noticed this problem since the first year I moved in. My floors feel like they are sagging throughout. It is 24' x 24, and is three stories, the first floor is the garage and separate hallway that has a stairway leading to the second floor. This floor is completely open except for a bathroom and staircase on one side leading to top floor. The joists run East to West, and if you go to the East or West side of the house, the floor is noticeably going down as you get about a foot from the wall(I have 30 gallon fish tank on this wall and it is noticeable leaning forward). Also, if you go to the North or South wall, it is completely flat. This is true on both floors. Also, on the main floor, if you are walking on the spot where the below you is the wall separating the garage and the hallway on main floor, you can feel it immediately go down, right after the support, like there is a bump in the floor.
I am no handyman, but it seems like as soon as the floor leaves the support area, it goes down, then levels out. My house is under structural warranty, but I dont know if this would be covered, and they want to charge a fortune to send someone out. Does anyone have any ideas why this would be happening? Everyone tells me it is no big deal, but it seems like one to me. Can this be fixed? Will this cost a fortune? Like I said my floors are open, so there really no room for a support.

[thedcdude]

Do you have any drywall cracking or doors not closing right?

[pushkins]

Run a string line from one side tight up in a corner (wall to ceiling corner)to the other side tight in a corner (wall to ceiling corner), make sure you pull it tight. Ideally there should be little to no gap the entire length, what size gaps do you see ?

[rmw]

if the floor is saging you should hold your string line down with a 2x4 block between it and the cieling and then measure along the string it should be the same thickness as the block if it is less the floor above is saging if it is more the floor above is crowned up.