Hi @PhiloEpisteme - my pfSense box is actually based on the Sumpemicro 5018D - F8NT 1U barebones system:
https://www.supermicro.com/en/products/system/1U/5018/SYS-5018D-FN8T.php
I believe they also make a stand alone or desktop version of this as well (i.e. with the same CPU). With respect to noise, I would not call this system quiet, and the primary reason for that is of course the small form factor. With a 1U chassis you are limited in terms of the types of fans you can use and to get any decent airflow you'll need several small fans operating at quite high RPM's (which means more noise). While this system doesn't sound like a jet plane taking off, one would definitely notice the noise in an office setting. I haven't measured the power consumption on just this system specifically (only on my entire network stack), but with a CPU TDP of just 35 Watts it will be on the lower side. Consider also that the CPU wont' be running at full speed the whole time (unless the firewall is consistently loaded down), but any expansion cards you add will contribute a few extra watts. If you are looking to build a system with this CPU (or similar) it might be a good idea to just get the motherboard and CPU combo and run the whole setup in a larger (2U or bigger) case, which would allow you to use bigger fans.
Now having said that, given that your use case involves wanting to utilize 10Gbit speeds between subnets, I would recommend looking at a higher frequency CPU than the Xeon D's as @stephenw10 already suggested. The quad core Intel i3-8100 or newer generation i3-9100 would make good choices and are decent bang for the buck IMHO. Couple that with a solid motherboard (that has appropriate expansion slots), a 4 port 1Gbit NIC, and a 2 -4 port 10Gbit NIC and you'll have powerful system that will also handle OpenVPN quite well. The i3's I referenced do have a little higher TDP (65 Watts) but again, unless the firewall is loaded down the entire time, the CPU will scale back the frequency and power consumption will be lower on average.
I hope this helps - please let me know if you have any other questions.