The screaming sound you hear from the RR Dart and Garrett turbo-prop engines is the compessor. These engines have a centrifugal (centrifical) compressor. Have you ever noticed the scream from the APU on B-737, Electras, Hercs, CV-580s, etc? These engines all have centrifugal compressors. The wonderfull Allison 501 series engines have axial flow compessors.
With a centrifugal compessor the air is drawn into the centre of the compessor (a rotating disk with radial vanes) and slung outward by centrifical force, then is compressed in the plenum as more air is forced in from the compressor. if it has a second compressor stage it will flow from the plenum into the centre of the second stage compressor, then get flung out again into a second plenum then on into the burner section where fuel is added and burned, the expanding gas flows out to drive the turbine(s). In order for a centrifugal compessor to work it has to turn a very high speed, maybe 30.000rpm, causing the well known scream.
With axial flow compessors, there will be multiple stages of blades (airfoils) mounted on a shaft that rotates. In each stage the blades are shorter than the stage before and the compressor housing also narrows down in order to maintain the blade tip clearance and therefor maintains the compession. Axial compessors are able to run at lower speeds = less noise.
Allison 501 series engines have a 14 stage compressor and the engine can operate at 2 speeds, normal is 13,820 rpm and low speed ground idle is in the range of 9,900 to 10,300 rpm.
Now I will confuse the issue a bit, the RR Dart, Garrett TPE, and the Allison 501 all operate at basically 1 speed and are primarily controlled by prop pitch, with some sort of associated fuel control to ensure that the correct amount of fuel is available.
Then there is the Pratt & Whitney PT-6 series engine that has both axial and centrifugal stages in it's compessor.
This is a Sesame Street version of gas turbine engine compressors, RR published an excellent book on gas turbine engines called "The Jet Engine".
I hope this helps.