Yeah, I truly miss those "2CY" air compressors on the R-10's that the cars originally made, and they were, ahem, "fun" to listen to. If any true R-10 fan wanted to hear them going off at full blast in the mid-1970's at least, one of my favorite places to listen was at 207th Street on the IND "A" line with each ten-car R-10 consist occupying each track.
I would verify that the G.E. R-10's air compressors did run slower at times, and I can particularly recall that fact on two cars: #3292 sitting at Rockaway Park station, and #3335 in storage at Concourse Yard, both in the late 1970's. Each of them on those units sounded so very rough and not necessarily in the same tone and pitch as the other R-10's I had been normally accustomed to hearing.
I was never that crazy about those NYAB rotary screw compressors in the first place anyhow, and you are right about being in most cases annoyances as mentioned in your post. It did not help matters along with getting a green R-10 car with spotted wheels as a deadly audio sounding combination.
Thanks to a fellow railfan colleague, I have such a situation like that, which was originally recorded in 1988 on an audio cassette sent to me awhile back. As I hear the tape of the "C" train arriving at Beach 105th Street-Seaside, one GOH R-10 car's NYAB rotary screw compressor did sound like it was really leaking something as well being having the effect of an electric hair dryer turned up to high, plus the flat wheels to boot.
Finally, it is true that there were a certain number of green R-10's that kept their original 2CY air compressors at first, even after their initial overhaul. As now getting used to hearing the noise of the NYAB units on the other cars giving out their usual stuff audiowise, I was pleasantly surprised while waiting on the downtown platform of 125th Street in 1985 that this one GOH'd R-10 that came into the station (I think it was car #3170 or something) was sounding off with its original 2CY unit.
-William A. Padron
["Fulton-Lefferts Blvd."]