Super quiet. The only sound is really just from the vacuum of the deck.
The speed of the blades does not do justice on the video!
24v system.
42″ deck.
6s Brushless Turnigy 174kv.
Hobbywing 150A ESC.
, https://i.ytimg.com/vi/zPzSrTBtdvs/hqdefault.jpg
source
What is the RPM of the pulley/blade?
you're going to lose quite a bit of efficiency though that belt.