After upgrading all the S5's and scoping out the temp charts, I picked a miner with the highest fan speed and temperature and took it offline. (Note to Bitmain: add a cool-down cycle that is triggered by unplugging the net cable).  After an air blast in the garage to get rid of the dust, proceeded to take off the fan and end brackets using a folded piece of cardboard to separate the fins on the heat sink.  Next, I removed one of the two ASIC boards with the dozen or so spring screws (clever!) and noticed that the thermal compound on the 1384 chips was mostly missing !  Maybe an unfortunate over-heat in a prior life, who knows.  After cleaning up the heatsink and board with contact cleaner followed by iso alcohol, I applied some GELID Solutions GC-Extreme thermal compound to the chips, installed the first board, ditto for the second one.

Bottom line, new thermal compound resulted in 600 RPM fan reduction and 8-10 Deg C in temperature !  Took about an hour for the first one and $5.00 in Gelid compound.  Happy camper here !