Since the system was running relatively cool, my next thought was to see what noise benefit could be achieved by using the low speed in-line resistors that came supplied with the two 120mm AcoustiFans. If fitted between the fan and the motherboard, these devices will drop the voltage from 12V down to about 7V and as a result lower the rpm and ultimately the noise. The downside is that the temperature inside the case might get too high due to the lower airflow. The only way to find out of course was to try it.
What a transformation. With the in-line resistors installed, the noise from the case fans was reduced to a very faint hum and our PC was now virtually silent. The only other sound that could be heard coming from the PC was the barely audible noise of the PSU and CPU fans. Because the rest of the PC was so quiet, the hard drive seek noise was now a bit more discernible than before, although not overly so.
The measured noise levels from the PC were now 30dB(A) or less, which was more than a 6dB(A) improvement from before. I dare say that under normal circumstances, this PC would be inaudible over the general background noise that can usually be heard in most bedrooms or living rooms. Interestingly, the speed of the case fans was now too low for the BIOS to monitor how fast they were spinning.
The only price to pay for such a dramatic reduction in noise was a slight rise in system temperature, which was now about 32 degrees C. The CPU and hard drive temperatures however remained unchanged from before.