My wife and I have a 1991 Mazda Miata we got new in the fall of 1990 “he” has a little over 100K miles on “him” and has been my wife?s car so it is in extremely good condition. 
Always garaged, regular maintenance, ect. (We named him Oliver because we had to eat gruel to afford him).
I am trying to fix what I call the Mazda pickup truck rattle. I call it that because I have noted that older Mazda pickup trucks make this noise.
I have convinced myself that it is a exhaust noise, because just like the old Mazda pickup trucks our Miata makes this noise as the car is slowing down under back pressure.
It happens at 2,000 RPM in 2nd gear and 3,000 RPM in 3rd as we down shift when slowing down.
It is not noticeable during acceleration or at speed.
I have stumped my dealer and I can?t seem to find people talking about it on the internet. (which is very odd)
During one visit the dealer removed all the heat shields but that wasn?t it.
I am about ready to start replacing exhaust parts but hate to waste time and money shot gunning the problem.
Take a rubber mallet and rap on the muffler. If it rattles there’s a broken baffle inside. Rap on the catalytic converter. If it rattles the substrate inside is broken.
Tester
Ditto. I’ve personally had a rusted out baffle in a muffler, and this was exactly what it did…rattled at a resonant frequency.
Thanks for your post, the rubber mallet test was one of the 1st things done. But just on a whim after reading a post on this site about the heat shield on the exhaust manifold I thought I would give a peek under the hood. While the exhaust manifold heat shield is solid as a rock I did find a little heat shield down toward the bottom of the exhaust manifold. It is made on very thin material and only mounted at one end, and sure enough if you reach down with a very long arm and fingers and give it a pluck it rattles in a very familiar sounding way. Next step is to immobilize it somehow and see if it is indeed the culprit.
Possible culprit (see attached)
Unfortunately that was not it.
Even though as you can almost make out there was a crack that extended almost all the way across it.
It only took a few wiggles to get it to fall off.
So is it worth keeping a 1991 Miata all stock as a collector?
Because I am thinking about stainless steel headers, and exhaust.
I had a buzzing/rattling on my 90 Miata that turned out to be a broken up internals of the catalytic converter. I also had a buzzing on the oem header heat shield
Keeping a 92 all stock as a “collector”? My guess, yes and no. Not with over 100,000 miles. I think it will be like the early Mustangs, a rare all original car is going to be nicer because so many get mod-ed and down right hacked up. Doubt they ever will be real rare and expensive. Keep the original exhaust around if you want to retrofit someday.
Good place for info and vendors… http://www.miata.net/