Knocking/rattling rear of engine

[Lucrative]

I have a 2000 7.4 MPI Mercruiser w/ Bravo 3 outdrive. I just finished changing out the bellows and the gimbal bearing, and when I am sitting in my driveway, I hear an intermittent knock from the rear of the engine. I greased up the u joints and everything well before I re-assembled. It appears that the shaft that runs into the rear of the engine is running in alignment, but there is movement from the larger, circular part where the shaft inserts into the engine. Don't know what this part is called.... Also, it happens more at advanced rpms.

I am hoping it's ok, because I feel like it used to do this a little before I did the work on it, and I think there might be a few causes, but not sure.
1. The gas in there is kind of old and not the best. I changed out the fuel filter recently when I changed the fuel pump and there was a little bit of water in there and some particles swimming around. Hoping I can just run this gas through with some stabil and maybe seafoam and it will work itself out, but I'm not holding my breath that that's gonna be the solution.

2. I changed the spark plugs as well. I used the NGK ones that were recommended, but the ones in there previously were GM ones I think. Could that have thrown off my timing at all? If it did, would it cause this particular vibration/rattling at the rear of the engine like this?

Thanks!

 

[Bt Doctur]

Called the coupler, its bolted to the flywheel . A common cause of knocking could be that the gimbal bearing is not fully seated in the bore .
and the corners of the input shaft are hitting.when the drive is turned left or right.
Another noise generator are the exaust shutters, if there the metal type . they will rattle in the exaust pipe

 

[Lucrative]

So, I forgot that I had my engine trimmed all the way up and that's what happens when you do that.....oops. I trimmed it down and the knocking went away.

 

[achris]

I have a 2000 7.4 MPI ....

2. I changed the spark plugs as well. I used the NGK ones that were recommended, but the ones in there previously were GM ones I think. Could that have thrown off my timing at all? ....

Different brands of spark plug can not change the timing. Even putting the wrong plugs in will not change the timing, even on a points engine. You have MPI, the ECU ('computer') sorts out the timing based on a variety of sensor inputs....

Chris.........

 

[Scott Danforth]

So, I forgot that I had my engine trimmed all the way up and that's what happens when you do that.....oops. I trimmed it down and the knocking went away.

Thats ptobably the u-joints

 

[thumpar]

You say the shaft appears to be in alignment. Did you use an alignment to to make sure? If it is not aligned you can lose the coupler and then you have to pull the motor.

 

[Oshkosh1]

Thats ptobably the u-joints

What he said!

 

[redneck joe]

So, only happened when in gear and up, yes ujoints