Hello Brad, I've had a couple here for about 15 years, one in regular use. To confirm the fan is on continuously, I've never noticed it off after hours of use and it isn't that quiet. Great generators for low level setting accuracy and low leakage.
Some signal generator fan drivers are complex, don't know if this is one of them. Example is the iconic HP 8640A & B, zero non harmonic spurious except for AC line, low phase noise and lockable. Wonderful things.
The full service 8657B manual pdf is available on line. Regards, Martyn.