I have a 1.6 Mk1, 1991 year.
When I am driving and come off the gas, there is a definite noise which I would describe like metal being machines, or a loud whining.
It seems to be to be coming from the differential - I could of course be wrong, but it certainly seems to come from behind me.
Only occurs when the car is slowing with the clutch engaged.
We appear to be getting more reports of Diff problems on the MK1.
Those cars have smaller physical sizes diffs to the later cars.
I have the same problem, drives me crazy…doesnt seem to cause any issues. Actually seems to. Be worst at 3000, rpm.
Could be a transmission backlash noise. Mazda issued a TSB on it saying that it’s normal. Try a search.
I had this on my 1990 MK1…I bought a diff and prop and gearbox to cure it then found it
It turned out to be the bearings in the propshaft that had gone dry. Try getting some gearbox oil, or spray grease onto the bearings… correct method is remove the propshaft, give the joints a tap (to release any stuck needle bearings) and grease
Hee’s a bit of light reading for you, from my archives [you may need to sign up to the Miata forum to read them
http://forum.miata.net/vb/showthread.php?t=320965&highlight=NA+transmission+rattle+decelerating
http://forum.miata.net/vb/showthread.php?t=436486&highlight=NA+transmission+rattle+decelerating
http://forum.miata.net/vb/showthread.php?t=53987&highlight=NA+transmission+rattle+decelerating
Another one for the prop here. Had the same problem on the Eunos. No noise on acceleration but a proper metallic squeal when lifting off the throttle. Make sure you get the right sized prop.