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.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013