So is it the aircon when turned on that is causing the fans to go noisy or the hot water system when heating the water?
If possible check the premises' fuse box to see if the aircon and the water heater have separate fuses or breakers. It should be marked as to what fuses feed what circuits.
Most gas water heaters have electric power connected to them to power the igniters, solenoids and the controls for when the the heater needs to be turned on. The gas is only to heat the water.
Usually ducted aircons (is it evaporative or refrigerated?) need a 2 phase power supply meaning 2 fuses (or breakers)but regardless of type the aircon and the water heater both should still have separate fuse(s) or breaker(s) and not be connected through the wall outlet or light fuses or breakers.
It may be that if one of the aircon power supplies or the water heater power supply is on the same supply circuit as the fans it is dragging the supply down causing the fans to not operate correctly. It may be that there are too many appliances etc on one particular phase or there may be a problem in the aircon or water heater circuit
You may have to isolate the cause of the problem to either the aircon or the water heater first to get to the bottom of it.
In any event since you are only renting the property, this is the landlord’s responsibility, assuming that everything was in place as is before you started renting.
Contact either the real estate agent’s property manager who you normally deal with and tell them about it or if you deal with the owner direct tell them. It would be in their interest to know about it especially if it turns out to be an electrical problem