Trains at Idle - Trains Magazine

Thanks, I was going to post a similar question earlier with one additional observation.

I've been train watching & train chasing since the 1970s (I'm 40 now). Managers had no problem with me coming into the big Uceta Yard, in Tampa, on my bicycle to take pictures as long as I behaved (gosh I miss those days!)

Up until the early 90s, whenever I came upon idling locomotives to photograph I would hear the long "tssssshhhhhhhh" from the unit or units every few minutes or so but I never heard "rapid clicks". These were SCL U-Boats, GP40s, SW9s, and occasionally Amtrak SDP40fs. I started hearing the "rapid clicking" in CSX units around the mid 90s. Was some type of upgrade made to the compressor systems that resulted in this?

The reason I ask this is because when New York City transit authrority rebuilt the "Red Bird" subway cars (now retired) back in the early 90s, the air compressors were changed to "rotary" type units that couldn't even be heard and the pleasant old "wump-wump-wump" sound was gone.