Steve gave me access to his code for the Fireboxes, but I gave up, as I wasn't getting very far.
I ended up using a splitter cable and a small potentiometer on the two fans on the cpu side and this reduced the noise quite a bit.
However, after doing an update from 2.1.5 I found that I was getting a lot of errors in the logs, relating to the NIC's. It's a while ago so I can't remember all that I did, but I believe it was due to an update in the BSD OS. I went back to 2.1.5 and the problem disappeared. I don't know if the situation has changed in the meantime, but I am looking at other devices that have Intel NIC's and may well replace the Proventia at some point.