susurrus or susurration is a very literary word for a whisper/whispering. The usual adjective would be susurrous or susurrant, rather than susurrential, but in any case it would mean “full of whispering sounds”.
I think it's a portmanteau of susurrus and torrential, which serves to invoke the force and volume of a torrential downpour to the description.
Most likely a corruption of susurrant. From susurrus, meaning a murmuring or whispering sound.
I believe that is his way of calling us gossipy ultracrepidarianists.
"An ultracrepidarian is a person who offers opinions beyond their own knowledge"