Nothing specific off hand, I used to work for a firm that did aviation noise analysis and this was something I heard a lot from colleagues at the firm and other firms. A starting point might be a couple of presentations from that firm that don't discuss the appropriatness of 65 Ldn directly, but discuss how that criterion is not always adequate:

http://www.hmmh.com/cmsdocuments/UCD_Mar07_Eagan.pdf

http://www.hmmh.com/cmsdocuments/DNL_History_Eagan_FAA_NER20...

http://www.hmmh.com/cmsdocuments/Eagan_Beyond_DNL65.pdf

I work in a semi rural neighborhood that's 2km from a regional airport (a bit off to the side of the approach path).

Approaches generally speaking tend to be much quieter than departures. What airport? Depending on a bunch of factors (noise abatement procedures, prevailing winds, approach tracks, fleet mix, etc) it's entirely possible it might not be loud where you are, vs another location. It's also possible that you're less sensitive to the noise because that you are working rather than trying to sleep (and also due to possible sound insulation measures as indicated).

Second anecdote, there was an airport downtown on the water [..] So on the launch day, before the sun rose,

Key word being "water"and "before the sun rose" - depending on temperature gradients (which tend to occur near sunrise and sunset) and wind direction sound propagation over water can have very unusual effects. In particular, if air temperatures high up in the atmosphere are cooler than air temperatures just above the water, sound waves can "bend" upwards to that folks on the ground will experience lower SPL's compared to what they might hear when the air temperature is more homogeneous.