Had #8 piston let go sounded like it was in the pump.
U-joints? front bearing? unhook the driveline and see if it goes away.

Daniel
Mash on it

I'm assuming you heard it at idle which is a fairly normal condition when the splines have worn a bit. Check your u-joints to rule them out as the cause. If all is good then run it.

X's 2 on the drive line as roost said , "machine noise" (stacato grumbling/rattling) is common in jets at low rpm/speeds and particularly running out of the water with no load on the impeller (not recommneded BTW) , this is also made worse by heavy impellers like stainless and bronze , things like "lumpy cams" and wear in between the male and female splines at the jet input can also aggravate the issues.Unless it worsens , vibrates when running or makes noise above idle (in the water) its not likley a problem beyond the annoyance  Tom