[stevula]

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”.

[ex3xu]

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.