While flushing my engines one at a time, I heard this noise coming from the transmission area. Sounds like a knock or maybe even grinding. It seems to go away or lessen in gear but the load on the engine is increasing, and the engine noise may just overwhelm the sound. It is hard to hear with both engines running. This was right after starting the engine so still cold. Also, I checked the fluid levels, and they appear to be in the center of the suggested range on the dip stick after the transmission and engine were up to temperature at 155ish degrees F. then turned off engine to check the fluid level. Looked at Android App for Fox Marine app for my Engine gateways and nothing seemed wrong but, the app does not present a full set of data like the Rinda Diacom, but no errors are logged, While I did not have time to do further tests, I thought maybe someone in this group would have an idea of next steps beyond the below "next tests".

1999 Carver 406 w/
1999 MerCruiser 454's with 1k hours Port Engine SN OL303934
Velvet Drive Transmissions, Model 20-01-005 direct drive - 2.455:1 ratio

Just spent 85 hours of run time down the TN River system to Mobile Bay AL 3/27/25 - 4/6/25 with no issues

One Drive link to Video file with audio.
vhttps://1drv.ms/v/c/9f1d79bbadce17c5/EX5EtKJ3xiNNhOo3y5MD_RsB4o8OY3Gn61kqTuyju5B8EQ?e=iu0HnC

My next tests will be:

Put on Rinda Diacom diagnostics software and capture all data to engine running info
Take the boat out into open water - Go below and listen while under way