S
SimonC
Thanks for the advice re cleanerAcid is only going to be much use in cleaning things like cement residues off. As this floor had been covered with carpet then I would guess that you have general dirt that has built up underneath it over time as well as the residues of old sealants/polishes/waxes (which would not be cleaned off using acid). I would recommend that you try an good deep-clean/intensive tile cleaner (correctly diluted) on the surface to start with; if this still leaves surface-polish like residues then try something like wax-off.
As for the crunching sound I think this will be due to some of the tiles being de-bonded from the substrate but I doubt that this is caused by the acid clean. Many of the old floors that I have seen have had some de-bonded areas. Sorry I've no really helpful suggestions other than if really bad then lift and replace the affected areas.