One problem related to differential unit has been reported for the 1999 Mazda Miata. The most recently reported issues are listed below.
Whining noise especially in 3rd gear at 29-32 mph that is very irritating. Very loud whirring noise at 5900 rpm in any gear. Dealers alternately say they can't hear the noises or that they are normal. One mechanic at morrie's said "I used to have a truck that sounded like that. Maybe yours will go away if you get more miles on it. " the 5900 rpm noise decreased by 2/3 after changing to synthetic transmission and differential fluid but there is definitely a problem. The last dealer I took it to said he'd call back in a week. That was three weeks ago. Mazda USA said they'd call back. That was a week ago. I'd like to get this fixed so that I can enjoy making my $450 a month payments for 5 years a little more.