Kane, I had the same issue when I did a very thorough winterizing last fall. Found all four bolts were loose. This wasn't a cause of vibration for me, at least I didn't notice a vibration, but this sounds like it is a fairly common occurance. I replaced my bolts once I found this, just to be sure they were in tip top shape. I found that all four had been worked back and forth enough to flatten the threads that were inside the coupling. Just not a good feeling for sure to find those all loose. Let us know if that cures it for you - and luckily you've got a back up plan if not!!
If you do decide to replace these bolts (I would recommend it) make sure to use some anti-sieze on the stainless threads. I had one gall on me enough that I had to cut it out... not a fun thing to have to do with the limited space. I got my bolts from McMaster - so maybe lesson learned.
Good luck man!