Sometimes the hissing noise isn't gas but just air, so you turn it down and think you hear gas coming out but in actual fact is is mostly/just air... Try giving your oven a thorough clean with brillo pads etc, then buy a can of compressed air (you can get them from pound shops) and just blow this down the offending hob(s)
If all this fails then you may need to consult manufacturer/retailer/repairman.
Hope this helps.
Kind Regards
J-J12
Our oven does this too. It happens because the thermostat cuts off enough for the flame to go out. Open the door first to let the temperature drop before you turn it down.