I had the Paragon P45'S bolted up behind Ford 427's and my starboard engine would have the same problem from time to time. There is a brake band around the planetary drum that engages reverse and causes that aweful whining sound when the gears inside start rotating against the drum housing to produce reverse rotation.
The brake band is engaged by a plunger in the control housing. This plunger has a rubber O-Ring on it that likes to pop off if you power hard in reverse. It is easy to replace by removing the control assembly cover. If I remember correctly, there are 6 or 8 screws holding the cover on.
I kept spare O-Rings onboard but switched to diesel with velvet drives a few years back. In the US,Transmissions Marine
Inc can get you just about any part for the P series Paragon transmissions. I rebuilt one myself about 8 years back and they are pretty simple to work on. Ran that tranny for years without problems.