* You've tried another shower head with the same results.
* Is the sound a whistle or a hum?
* If it's a hum, or the water pipes bang or make noises at times, then there's another possibility. Your toilet! It could be any toilet in the house.
* We've discussed this phenomena on this for'em several times. When the shower is turned on, it causes water pressure to drop. The toilet reacts to this drop in pressure and the gasket on the toilet fill valve starts vibrating causing a hum or making the pipes start banging.
* Far out? Yes, but it happens. Plumbing is more complex than I imagined! Shut off every toilet water supply to see if this the culprit.
* Good luck... Adios... Jasper.