Rear differential whine and mounts

A whine from the NC's rear differential is a known fault: Mazda issued a service bulletin for it. This page says what the bulletin says, what owners reported, and what differential mount bushings are sold. It is not a repair guide.

How to read it. The owner accounts are from Miata.net forum threads that I read in full on 1 October 2026, dated below. They are individual owners' experience, not tested by me, and posters sometimes disagree. There are no prices on this page.

Mazda's bulletin

Mazda Motor Corporation's Service Information R066/14, "Whining Noise from Rear Differential", issued 28 August 2014 for the MX-5 (NC) worldwide, is in Miata.net's collection of service bulletins (the PDF). I read it. In my words:

• The symptom is a whining noise from the rear differential while driving.
• The cause Mazda gives is an improper tooth contact pattern in the differential gears, from abnormal wear, which comes from the gear-tooth lapping compound used in production being left beyond the allowed amount.
• The production change improved the control of remaining compound. For the US it starts at VIN JM1NC****** 233232, built 29 May 2013; the bulletin gives other starting numbers for other markets. The cause applies to cars built before that.
• The repair it gives is to replace the rear differential assembly with a new one, per the workshop manual. It does not mention changing the oil, and it is a bulletin for dealers, not a recall.

What owners say

From the September 2014 thread where the bulletin was first posted:

• What it sounds like. Posters described a low-pitched hum or whine that follows road speed, not engine rpm, most noticeable from roughly 40 to 80 km/h; one said it hummed at a steady cruise and quieted when he added throttle. One owner said it is much higher pitched than a wheel bearing, and that a mechanic can trace it by running the rear wheels under power on a lift with a stethoscope.
• Warranty. Several owners said their differentials were replaced under warranty, in some cases within the first thousand miles or after the dealer had a regional Mazda representative listen. One poster noted that a replacement from the same period could have the same fault. The poster who brought the bulletin said that few owners will need a new differential and that having a noisy one in the warranty period is what matters.
• Oil changes. Several owners changed the differential oil early, at 600 to 1,000 miles, and one found a clump of metallic residue on the magnetic drain plugs of both the differential and the gearbox. The poster who recommended it also said an oil change will not repair a differential whose gears or bearings are already damaged. This is owners' reasoning, not something the bulletin says.

Differential mount bushings

A different thing: the rubber bushings that hold the differential in the car. In a July 2019 thread an owner swapping in a factory limited-slip differential chose Whiteline bushings (75 durometer, about the middle of SuperPro's 70 and 80 options) and said getting the old ones out took three hours. Posters named Energy Suspension, SuperPro, PowerFlex and Whiteline as the options. One said stiffer mounts can reduce wheel hop and might improve shift quality a little; another had fitted 70-durometer SuperPro bushings on a PRHT car and could not hear any added gear noise. They are not a cure for a whine.

Whiteline and Energy Suspension also sell NC differential bushings, per the thread and search results; I did not open their pages.

Not covered yet

• Sellers of a new or used differential, and who will replace one outside warranty.
• Whether Mazda extended coverage for this fault on later cars, and the warranty terms; ask a dealer.

© 2026 Fischer Interactive LLC. Written for the Ref Sensor app's Miata users. The seller's page is the authority.