Our down stairs showers are back to back. One shower has recently remodeled and drain location slightly changed. When shower is run in remodel unit you can hear the drip and flow in the adjacent shower/tub. Also when the toilet is flushed next to the remodel you can hear the water flow and echo in the adjacent tub.

Sounds like the trap on the old tub has dried out. Try dumping some water down the drain of the old tub and see if the noise disappears. If not, something really got messed up during the new install. Please let us know what you find.