two showers whinning (or loud hum) on startup, then stop

Hi,
I have two top floor showers that both whine or have a loud hum at start then stop within 1 min...OK after that:

-> They both whine independently of each other.

->One has a new shower head the other does not.

->The house is approx. 18 years old.

->Both valves are DELTA

Are these tub/shower units? Or dedicated showers? If a tub/shower, and you have a diverter tub spout, those are notorius for making some whines/whistles, etc. Usually, the only way to fix that is replace the spout.