You can and I have seen the aluminum splines strip out the inside of the coupler with out spinning the rubber hub in a coupler.

same thing
 

Ive also stripped the splines on an alpha coupler , took me 2 days and the drive on and off 3 times before i figured it out , if you can get your head and a flashlite back in there far enuf it will be obviuos ( aluminum all over the shaft where it slides into the coupler ) other possiblity is vertical drive shaft sheared with same effect ( the broken ends will still rub enuf to spin the prop out of the water ) good luck with it Bruce

Same thing happen to my 42 with the stock hp 500's. On a poker run came to a stop and the drive would not go into foward. Finally went into gear so I figured the cable had stetched no big deal. About 5 miles later the drive started making that same noise you are hearing. I restarted the engine and heard some banging like half in gear. It ripped the floor out out of the upper housing a destroyed the drive. Bought xr's and replaced of them

Terry

1. Drain gear lube and inspect
2. If gear lube looks bad, their's your problem
3. If ok, Pull prop of and inspect male and female splins
4. If all ok, Pull drive, It has to come of anyways so just do it
5. inspect drive yoke and drive coupler.
6. By this time you will find the problem.
Jr.