RPM does not generally fluctuate in flight, or on the ground for that matter.
For 'fan' RPM, 100% N1, is somewhere between 2000 to 4000.(aka LP stage)
For 'core' RPM, 100% N2, is somewhere between 9000 to 14000.(HP stage)
The 3rd spool (IP stage) RPM would be in between the LP and HP nominally around 7000.
These are ballpark figures.
All high bypass jet engines make this roaring noise due to the blade tips of the first N1 stage (The first fan blades you can see from your seat). At take off this noise is generated by the blades tips which are rotating at 75-95% of maximum power, nearing or exceeding the speed of sound. This creates a MILD continuous sonic shock waves (supersonic bang) ...