Micro cracking floor/creaking

[technoloops]

I have a 2022 Miller Homes house which has micro-cracking/creaking from one of the bedrooms.  The floor is a cassette with 18mm OSB and 22mm (I think) chipboard for decking.  I know the cassette is built into the block work and the stud walls build on top of the cassette. The floor feels bouncy.  The engineered I-Joist runs from the external wall to the internal centred load bearing wall.  The micro-cracking and creaking seems to come from area around the external wall and internal wall.  The joists are at 600mm centres.

The developer came out and screwed the decking down around the areas above where the noises emanate but this did not work and the floor still feels bouncy. 

I have read the various threads about this issue and understand the NHBC was involved in research into this:  http://www.brand-newhomes.co.uk/Joint-statement-on-cracking-sounds-from-ceilings.pdf

1. The report states dot and dab drywall attached to the internal block work.  I think my internal walls downstairs is wood stud work with plasterboard.  So how what would the fix be to remove contact of the drywall from the underside of the joists?

2. Regarding the floor bounce, is the correct fix is to install additional joists? Would adding herringbone strutting (as indicated in the NHBC regs) help?  Since the cassette is built into the block work, how would additional I-Joists be installed as they are not on hangers?

3. There is a patch in the ceiling that was exposed to investigate ticking plumbing pipes, I had a look at the joist and they are 220mm tall and the flange is about 42mm.  I have looked on the joist manufacturer's site (James Jones JJI) and the specifications of the joist is correct to be used 600mm.  However the closest spec joists is 220mm x 47mm.  So that would mean the flanges of the installed joists are about 5mm under spec even though the joist height is correct.  Would these 42mm flanges affect the joist stiffness and deflection? From what I understand it's the web that contributes to joist stiffness and prevents deflection and the flanges prevent twist?  The room depth is about 3500mm.

https://www.jamesjones.co.uk/products-and-services/engineered-timber/jji-joists/interactive-span-table

Thanks

[New Home Expert]

I would suggest the joist centres should be closer even if the design is correct. The larger the flanges the more load bearing (less bouncy) the floor. Herring bone strutting may help and may have been missed.