Since installing a new dual-flush toilet we now hear a loud gurgling noise in the kitchen sink which is about 15 feet from the toilet. Any idea what's causing this and what could be done to stop it?

we now hear a loud gurgling noise in the kitchen sink]
The sound you hear is the system attempting to vent through the kitchen sink trap. This indicates a blocked vent. The solution would be to snake the roof vents. Put out enough snake to reach the base and 15 feet more. Good luck, Tom