Is the water heater gas or electric? and 'small' as in r2d2/beer keg sized?
Gas water heaters gurgle/pop/say hello when sediment builds up inside them and only when the burner is on. The sound is caused by water boiling at the bottom of the tank chamber as the flames impinge on it and the sediment prevents the transfer of heat through the water heater jacket from happening evenly. It can also cause the heater to fail prematurely so best to address it soon. Solution is to flush the sediment out by opening the drain valve after attaching a garden hose to it. More common with houses on wells but I've seen it happen with city water systems too. Electric water heaters usually have the heating elements suspended somewhere mid tank and not at the bottom.
I'd be surprised if an electric 'small' water heater was tied into an HVAC system with a heat exchanger coil...not enough BTU's available to warm a snails butt but homeowners and even the occasional plumber do strange things when chasing efficiency.
Why it happens when the AC is on is a complete headscratcher to me... quick experiment would be to run hot water at a sink without the AC on and see if the noise is present or not?
