I have heard this several times that the CRJ cockpit is much more quiet the ERJ (CRJ200/700, ERJ135/145 not E-jets). This came from an ASA captain. I too have noticed that the ERJ guys are always wearing David Clarks or some other large headsets for this exact reason supposedly. Yep, I've seen the CRJ pilots wearing the little Telex head phones.
Second, why is the 135/145 APU so loud! I HATED working around those things. I always tried to avoid the north end of D concourse for this same reason. You could hear the thing from halfway down D and when you drive behind it, it was this high screeching noise; almost deafening.