Honestly it's surprisingly quiet. Here is a video of my testing my V2403:
Project Felicity - 01 - Kubota V2403 Engine Test Run
What really surprised me about it was how quiet the diesel engine was with no exhaust on it at all. Also the longer I ran the motor the smoother the engine got as well. I did run it one more time where I revved the V2403 a couple of times and it really wasn't that loud again no exhaust at all, it didn't seem to hurt my ears at all when I was running it.
I've had V8 and V6 run really quick with no mufflers on them that were louder at idle than this engine was. So I guess I would say it's difficult to say preciously, but I figure once I get the engine in the TJ, and have it connected to a cat and muffler, it's not going to be that loud at all.
It's by no means a 4BT, those engines I believe are 750lbs and the Kubota V2403 is 414lbs, so you have around 350 pounds less running and trying to vibrate.
Another thing to consider is a 4BT was originally made from 1984 to 1998, the Kubota 03 series started production in the early 2000s, my V2403 is from 2007, so you are looking at 20+ years of new diesel technology.
Let me know if you have any more questions!!!!
Thanks
Grant