Tonight my toilet started making glugging noises and on inspection it was large air bubbles in the water ( which was at normal level), I flushed it and it filled almost to the brim before slowly returning to its normal level. Flushed it again and it worked properly, fast flush, normal water level. Now I have a problem with the water level in the bowl dropping after an hour or so to a level where air is being drawn through the bend and the toilet is noisy again. I have flushed it repeatedly but the water level in the bowl is still slowly dropping. The problem with the dropping water level first happend about 2 months ago, lasted an evening and has been ok since. I have no water leak from the actual toilet, the carpet around it is dry. Any thoughts?????