The JMC/Datech DS9238-12HBTL case fan specs are 1100-5100 RPM, 100 CFM and 51 DB at DC12V 1.50A. Does the fan normally/mostly run at 1100 RPM and only go up 5100 RPM when needed?
Judging by the specs, it looks like it starts at 1100 rpm - a nice slow speed IMO - and ramps up to 5100 rpm as it gets warmer. At what point the speed increases and how it ramps up who knows? If it's any comfort, most of the fans I know of increase the rpm relative to the temp - ie. infinitely small steps.
The relationship between rpm and cfm is complicated and beyond my grasp, but there is one. The charts on this page should be useful: http://www.cpemma.co.uk/index.html
The datasheet doesn't mention anything about thermal control. Can you see if the fan has a little thermister jutting out anywhere?
Does going with the same model fan except going from 1.5A to 1.7A make that much difference in the noise?
Prolly not very. That's only a 13% increase in current.
Is it a good idea to replace this fan with a Panaflo FBA09A12L1A, 2100 RPM and 42.7 CFM. It is less than half the CFM than the JMC and does not seem to be variable speed. But if the JMC is not at 100 CFM all the time does it really matter?
This may depend on how your fan is controlled. Is there a thermister somewhere?