I spent most of the weekend under the car, adding insulation and foam bits to various panels and heat shields that I suspected could be the noise culprits. I even added insulation next to the exhaust manifold's heat shield under the hood. Needless to say, no matter what I did, the noise was still there. I must have jacked the car up at least 5 times and done at least 5 test drives. I concluded it must be the drivetrain and not any harmonic vibrations elsewhere in the car.
Yesterday I had an appointment at Mazda of Escondido. Call it a "second opinion" on my car's drivetrain noise. They were more polite, detailed, thorough and overall more helpful than my visit to the first dealership. They test drove my car and compared it to other cars they had on the lot. They drained a bit of oil out of the tranny to inspect for debris. They said all was well.
They told me that the noise it normal. According to them, they compared it to other Miatas and they said the quietest one was a brand new one they had on the lot. They said that after some miles they start getting noisy because it's a "very aggressive transmission." They said, they had a used one with some miles on it, and when they drove that it, the rattle was even louder.
When I bought my car, it was new, but it already had 40 miles on the clock. Not 15. Maybe that's why it was making that noise from day 1. Given what they say, it sounds like I can expect it to get louder as times goes on. Not sure I like that.
I noticed this weekend, the worst rattle comes in around 2,000 rpm in 2nd gear when driving steadily. Also, when driving in traffic and when pulling away from stop lights and stop signs at normal speeds.
At this point, I'm not sure what to do. I would guess that with all the people here with these cars, if this was really normal, there would be more talk about it since I find it to be quite an annoying noise.
My car: The 1995 RallyWays Miata