Another one of these noises you can't figure out!

[Paul99]

230 1966 auto. 120k miles.   Now after lots of tweaks  it starts well runs well restarts well, drives well sounds great....... But.......

On  acceleration  it Sometimes sounds like the timing is out and I get that horrible rattle which says change down now!   I have checked the timing. Spot on, the vacuum is working on the dissy and running on 98 Ron fuel. 

The strange thing that a couple of miles later, same situation and zero rattle. Perfect.  Sometimes driving in 4th at say 40 mph, floor it but not kick down and zero rattle.  Other times just a slight bit of throttle and mega rattle as if the timing is way out   

Can't believe timing can change at a whim so something is causing it.  Not usual thing like exhaust rattle definitely timing type noise. Baffled.  Any suggestions? 

[BHap]

My exhaust heat shield recently came loose and sounded like an engine "rattle".  This is the long one between the exhaust pipes and the floor pan under the passenger compartment.  Three screws hold it on.  Worth a shot.