Loud Sound from shower after supply shut off

[Jmcc64]

I was attempting to replace a Moen 3600 volume cartridge but had to abandon it due to the fact that the collar nut is inaccessible.(that's another thread) So, I had to shut off the water supply beforehand. Now when the tub is turned on or the toilet is flushed, we get an ungodly howl from the shower pipe for a few seconds and water will trickle out the shower head The house is plumbed with Pex and the supply lines to the shower come out the top of the manifold (3/4" dia.) and there was no inline shutoff so that's why the main was shut off. So, is it air in the line? If so, how. I never got the old cartridge out (not even close).