Creaking - Unpacked Joists?

[stewarth71]

Hi, I live in a relatively new built house (10-12 years). When we moved in we noticed the floorboards up the stairs were creaking quite badly but it seems to have got much, much worse. I lifted the carpet, checked the floorboards, banged in a few nails into the joists but I've now been told that it may not be the floorboards at all (which seemed screwed/nailed down pretty tight) and may be because the joists weren't 'packed'. There seems to be a slight spring when you walk on the floor, suggesting that these people may have a point...

If they're right and the joists haven't been 'packed', what do I do and is it the nightmarishly big job I fear it is? The entire upstairs floor seems to have creaks in places...

Help....!