It sounds like rushing water in a white tube coming from my a/c / furnace unit that is in a closet indoors. What is it & how do I make it stop?

If it is a newer condensing heater, water will be running into the condensate pump. Sometimes you will hear bubbling. If the noise bothers you, put a piece of metal tape across the top of the "t" and cut a very small slit in it for air.

Does it rush for about 30 seconds and then stops not to restart again for 15-39 more minutes

No it is constant, doesn't stop. A/c & heat are not on...

And it is a really old unit

If nothing is running there is no reason for water. Are you sure it's the heater and not the PRV from the hot water heater?

I don't think so... The water heater is brand new & in the garage... it sounds like it's coming from the a/c unit