Official Honda service manual online subscription: https://techinfo.honda.com/rjanisis/logon.aspx
1 day for $25, 30 days for $125, 365 days for $1250.
This is what the Honda dealership technicians will have. It will probably have the best explanations and diagrams.
I have read that this online system limits the number of downloads per time period now.
UPDATE: I managed to diagnose and fix the problem myself after being turned away by yet another mechanic. The problem ended up being a loose serpentine belt tensioner rattling when the engine was under load. I removed the bolt labeled B in the attached picture and cleaned it up before reinstalling it and tightening the hell out of it. Sound gone :)
That damaged seal is the most likely source.
Replace it as the first move.
Water can follow weird paths until it gets to the lowest point so where you test may not be the route it takes.
Once we had to put a small apprentice inside the boot and close it (he had a torch). Then we soaked the car with a hose and found the source of the leak.