Whilst googling for information about the ZT exhaust system I found this very informative post from the 260’s blog so I have clicked their REBLOG button to add a link to it for further reading.
See …… https://two-sixties-blog.com/2011/07/12/proper-exhausts-for-the-260/
It describes how some of the early cars suffered some levels of resonance between 1500 and 2000rpm and how this was resolved on facelift cars.
My early ZT suffers some light resonance at about 1800rpm so it seems like that is “standard”.
MG ZT info mods and maintenance 