Dunno, I ve thought it might be something to do with how the cambelt is done. I ve had a 53 147 16v mjet, a 54 147 16v mjet and they were pretty smoothish, but I know of two 53 156 1.9 16v mjets that were both much louder and more rattly. I know engine installation can make a difference to noise, but I really cant see why it would in this case - the 147 and 156 are pretty much modified versions of the same car. And I ve got two 147 Ducati Corses and theyre both rattly like the 1.9 16v mjets in the 156s I just mentioned, which I think proves its not down to body style.
My 54 147 had the cam belt done at Autolusso, one of the two 156s had the belt done by the owner, the other by a "normal" garage, thats why I had a vague idea it might be the more experienced the garage is with JTDs might make a difference, or maybe the parts supplier, different belt maybe.
The Ducatis have a plastic intake manifold so I thought maybe that could be it - the heavier alloy ones might absorb sound better . . . but ALL 156s had the alloy intakes, so it cant be that.
So, dunno, the way theyve been used throughout their life ? (all mine had at least 95,000 on them when I got them, those two 156s I mentiond had 100,00 + on them)
Its a shame as the non rattly ones sound pretty good I think, not as nice as the 2.4 but still not bad.