my toilet makes a loud fog horn type sound when flushed ?

I live in a first floor modern flat with only one other floor above me. The flat down below me had problem with their water pipes for many months before it was repaired as no one actually knew which flat the water hammer was coming from and it affected all three flats. This has now been fixed and the tenants below moved out and the flat had then had been left empty for some 6 months. The day new tenants moved in my toilet started making a loud fog horn type sound when flushed but not late at night or early morning when nobody else uses water?? I normally get up for work at 5am and flush the toilet and makes no noise, but then when flushed at 5pm it makes this loud fog horn type sound, the noise seems to be at its worst whislt during 8am till 10pm and sometimes during this time it only makes a faint noise. Any suggestions???????????