I believe you haven't hit the temperature for the fan to come on yet.
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D-Link mentioned the temperature spread in the past, but I seem to recall it was higher than that. That's not all that warm, note the SMART readings for the drives will probably be cooler. Here's what mine is doing right now, it's in a basement closet.
System Temperature: 105°F / 41°C
The two drives are at 84°F / 29°C
The fan is not running.