Its not usually the joists, it usually flooring especially t/g 8ft x 2ft chipboard flooring which most builders screw down in the conventional way...Wrong...
to get it perfect when laying all joints should be glued and I would also recommend across the tops of joists, the creaking you here is each board moving individually, if this is a new build i would argue it with your builder for not following guidelines, no easy way out you will have to pull the flooring up on the plus side if he hasn't glued it it will make it easier.

We had a similar issue in a house build. We had Jj joist out and Knauf who supplied the plasterboard.

It’s an industry issue with Micro cracking. There is a number of reports on it and due to the chemical make up of plasterboard changing. When plaster board company’s redesigned the boards to make them lighter for H&S law so it was safer to carry and install. it changed the density of the board with more air in them. This resulted in what sounds like the plasterboard cracking. We had a the joist company and plasterboard do a underlation test. They both came up with same results. A 0.005mm movement which was way over the allowance. So the joist were fine. A representative from the industry told us that they had the same cracking noise on a house with a block and beam floor and resilating bars. They monitored the noise with sound testing equipment and found out that the boards were micro cracking when the underfloor heating was turned on thus changing the pressure slightly in the room, the heat rising as a mass made the boards crack.

Here is one link from Gypsum
http://www.gpda.com/technical/technicalinfo
but all plaster board companies have these PDFs no one admits the problem as they spent millions changing the boards make up and have endless stock still being produced. Best out come is to eliminate the joist company so it’s defiantly not that. Then research who supplied your boards and plaster. And screws. Knauf informed us that if we used there boards but not there approved screws they would not be responsible. It’s a very messy conclusion and journey to correct.

From someone who’s fought for there client to rectify such an issue we wish you luck.

I hope this helps

Kindest Joseph

Definitely the chipboard tongue and groove flooring causing the noise . It should not be nailed or screwed and should be glued down to the joists.
The joists shrink and then the boards are able to flex up and down on the nail causing the noise.

the joins in the boards should sit on a joist and they should be glued to the joists and to each other, if when the floor is taken up and there are no noggins to brace the floor joists they should be fitted this should have been done to pass building control regulations