JT - I believe Scissor Link and Heli-phile are on the right track, i.e. does the noise level equate to the observed N1. If you're sceptical about accurate N1 indications simply swop the N1 and N2 tacho-generators over (they're identical) and see what happens. You said you have access to another JR, then try that N1 indicator in you machine - do the easy things first. If the problem is not with indication, then I would look at the starter-generator. check that the three connections are tight (pull the rubber covers back one at a time and check the torque of the nut, also check that the large firewall connector at the Fwd R/H of the engine compartment is tight.
If all of this is satifactory, remove the starter-generator and examine the brushes; a) are they worn, b) are they in the guides the right way around (they can be fitted backwards), c) are the springs making firm contact with the brushes, d) is any brush jammed in the guide, e) what is the condition of the comutator etc. etc. By the way, what is the TSN/TSI/TSO of the starter-generator?
It's still a little too early to be considering a replacement starter-generator and way too early to consider a turbine change.
An unusual problem, let us all know what was wrong when it's all settled.