Creaking floor

[lize]

I have had an extension built - the bedroom floor creaks - the builder has been back and screwed down all the floor boards - no difference. They then lifted the floor boards - added more insulation (it was sparce), applied some sort of strip to the joists and then replaced the floor boards and glued everything down. It still creaks. They returned and on their structural engineers advice removed the screws, wrapped PTE(?) tape round each one and the screwed them back in. It still creaks. Our builder now says its shrinkage or the span. Their structural engineer came out today - didn't think that there was a problem with the joists into the wall or the hangers - acknowledged it is creaking when walked on and says the builder has done "virtually" everything they can. But virtually everything does not mean they've done everything - and unfortunately he would not elaborate on the what else remains that can be done. Please can anyone advise me - I would be grateful for any help.