The clanking sound, you probably will not be able to do anything about, short of getting a new chiller. I have known refrigerator compressors that have run that way for years-annoying for sure, but they still work.
For the fan speed, the only way to really change speed is to either change voltage (120) or frequency (60hz). Electric motors are built to operate at a designed speed and voltage (one very common speed is 1725rpm). Motors with multiple speeds (such as fan motors for window ac units) are wound and wired internally for those different speeds. High tech commercial fan motors have frequency drives that can ramp motor speed up or down depending on need. Adding a resistor, which would divide the applied voltage between the fan motor and the resistor *might* work to slow the fan down but that resistor would have to be very carefully sized, after knowing the amp draw of the fan motor, so that voltage would be reduced *somewhat*. The motor still might not work well and might not work at all at a reduced voltage. I know a little about motors, but can't say for sure if this way would indeed work. Such a resistor would have to be a ceramic unit designed to handle the current levels that would result from designing and building such a circuit.
The best/easiest way to reduce fan speed would be to spec out the motor size, including Frame Size (48, etc), shaft diameter, etc, then finding a similarly sized unit that operates at a lower speed than the one you now have. Fan noise is a function of speed and design (blade shape) and in small inexpensive fan units is not a priority for manufacturers-they just spec out what they think will work and buy the cheapest unit they can find that will meet those specs.
Based on my experience working with A/C, refrigerator, and assorted commercial refrigerator applications over the years, I beleive that slightly reduced fan speed would still provide enough air flow to remove waste heat from the condensor coils of your chiller but until you actually make the changes to reduce that fan speed, it is not possible to know if the noise would actually be reduced. Fan/blade design can be a "black science", done in the dark until actual prototypes are built and tested. This assumes of course that there isn't anything wrong with your chiller such as restricted airflow on the input or output of the airstream.
That, and $1.65, will get you a cup of coffee at Java House on Q St. in Wash, DC.
How about searching for other users of your model chiller and ask if other users also have the same problems with their chillers?
I have Current USA chiller for my tank and it is sooo quiet that it's hard for me to tell if it's actually running.
Sorry for the long and complicated response, but hey, I got that degree and I gots to do something with it!
