[NA6] Grinding noise from transmission - Acceleration/deceleration

[xeon8]

Hi all,

I know these threads are aplenty, but I haven't come across one (yes, I do use the search function) that pertains to my particular situation.

So I've recently purchased a '91 MX5 and I absolutely love it, but the one thing that is concerning to me is the constant grinding/metal on metal noise coming from the shifter turret area during acceleration and deceleration/coasting. The noise its prominent in 3rd, 4th and 5th. In my travels I've found a possible list of diagnosis' but they are listed for other, similar complaints. Since the purchase I've had done:

- New upper and lower shift boots (I've added a little more sound insulation myself to try and cut down the noise inside the cabin)
- New gearbox, diff and shifter turret oil (correct weights and GL ratings)
- New OEM clutch, new TOB and pilot bearing
- PPF realignment

(All heatshields are also in tact and have no debris and rattles)

Though the noise isn't driving me crazy, it isn't entirely pleasant either. I've had my mechanic (same guy who did the clutch) look over it and he reckons its just gear mesh noise which he told me is perfectly normal. I, however think its something else. The noise isn't just in the cabin though, I can hear it clearly whenever I'm driving against a barrier or other solid object - even at slower speeds/light throttle.

My last option/place to look is the diff itself. I know the 1.8 torsen's have the damper on it which perhaps could clue me into something? Any input would greatly help.

The video is at https://www.youtube.com/watch?v=ZUXso4LslCw& It's is in 5th gear, but 3rd and 4th behave the same. The boots were replaced after the video was recorded.

Nick.

[StanTheMan]

Couldn't listen to your video. Its set on private.
its a good chance the diff could be on its way out. Its the weakest link along the driveline.
But if its coming from your gearbox....not entirely sure

[NitroDann]

90% its the diff bearings are flocked. 10% its the trans bearings are flocked. On an na6 both are practically free so just buy a spare and when it leaves you stranded swap it out.

[slug_dub]

Please mark your video public so we can see it, but I bet I know exactly what that sounds like.

You could try changing diff bearings, trans bearings, geabox, diff, seals, boots, clutch, flywheel, realign the box to bellhousing PPF whatsit magic, or sacrifice a goat. I have never found mention of exactly what or why the noise is there but plenty of asking what causes it.

After 6 years of driving several NA's with the noise, I've given up hope of ever finding or fixing it and just live with it. It's loud inside and outside of the car and ruins listening to the motor and the exhaust, but hasn't made the car stop working.

/jaded mysterious gearbox noise put up witherer

[StanTheMan]

install torsen conversion

enjoyment qudrippeled

if that doesnt fix it
install Svt 1.8 with turbo & 6 speed

[KevGoat]

Don't think this would be the cause in this case, but in My NA6 the clutch fork use to rattle horribly and that sounded like mechanical grinding. Also use to hear it bounce back off walls. Use to fix with some grease every so often.

[xeon8]

Video has been made public, I mustn't have realized it was set to private.

Thanks for the responses though, I know the diff is the weakest link in the 1.6's so that's why I've sort of been clued into believing that could be the issue. The next step for me is to run it on a lift or on jack stands to pinpoint the exact noise.

[91BRG]

it's definitely the gearbox bearings, I had the same noise on my 91 when I first purchased it. Its on load acceleration and low rpm deceleration right? It's not worth it to repair the bearings in the gearbox, just buy a low km secondhand NB8A g'box and put that in. You can reuse the clutch and flywheel. MX5 mania did the work on my one.

[AlexMiata]

Hi all, just wanted to weigh in on this decel grinding/buzzing noise issue. To be clear we are talking about the same thing, this is a commonly referenced noise in many forums and it's commonly reported characteristics are like this:
1. Almost exclusively on decel (occasionally something similar reported on accel)
2. Almost exclusively in 2nd,3rd, 4th. (Very occasionally reported in 5th)
3. Stops completely on touching the throttle as power shifts from overrun to accel.
4. Stops with clutch disengaged (peddle depressed)
5. Noise comes in anywhere above 2.5K-3K rpm, and persists until rpm drops to arnd 2.2K.
6. It is most commonly, not always, first noticed after a clutch R&R - but no link to a clutch brand/type

People have tried everything: some crazy stuff.
Changing 3 gearboxes, all kinds of diffs, custom building clutch friction plate spring rates. Most often there's no or little change.
Sometimes it is "resolved" - or so people say - by increasing weight of MTX oil 80/90w or 90w, or shandying 50/50 with correct weight.
Some say PPF realignment has fixed it. These are the minority.

My story:'
I'm not going to change an MTX or Diff unless I am absolutely sure that is the source, and the change will fix it.
I get that it's an approach that some take, but I prefer to diagnose, confirm and fix.

I've used Chassis Ears (sensitive microphones) on various underbody locations to isolate the source.
In terms of the source: I have eliminated the diff, many positions on the exhaust system, mufflers, CAT, heat shields.
Bell housing area is not a prominent noise source.
Mid gear box at the bearing housing is a bit more prominent, but not excessive.

NOTE: Extension housing at the end where propellor shaft yoke inserts into brass housing bushing is loudest by far; really loud!

I've toyed with the idea of replacing the bush.
It does seem possible without an MTX out job, but it requires and investment in a rare $150 bush and a special extraction tool, also about $150.
There is absolutely no evidence of movement, play, or any leakage at my extension housing, or where the yoke enters the bush.
(Remember, people have changed MTXs and still had the noise.)

Anyway, I haven't gone this way yet, because I'm not convinced it's the problem.
But it does indicate that the noise is from the MTX and probably related to (not necessarily caused by) a combination of the bearing housing, the long output shaft and the extension housing bushing.

Why?

I've noticed that the almost identical 5spd M526 MTX in the RX-7 has and extra part.
On the output shaft, where it exits from the bearing housing, the RX-7 box includes a pretty weighty looking damper assembly.
My suspicion is that the damper was introduced to reduce/resolve inherent output shaft vibrations and resonance.

Who knows if I'm correct. I can't find any technical reference to this part or why the part was included.
However, if I am right, this problem/noise is a characteristic of the M526 MX-5 Miata version of the MTX.
There resonant noise may be related to a vibrating output shaft??
It it were a worn failing bearing in the mid-housing, I can't see it lasting that long without worsening and collapsing, but some people have reported driving 100K Km with no change to the noise.

In my case, I've tried the PPF realignment, with no change, and stapped up all heat shields.
I am in the middle of adding some Penrite friction modifier, noise reducer to the MTX, increasing the dose slowly.
So far it is not conclusive (frustrating) but may have made a difference to the rev range at whcih the noise is present.
I need to do a bit more driving to confirm, and add a bit more friction modifier to see if improvement can be confirmed.
For obvious reasons (COVID 19 restrictions) I'm not driving much these days.

Interested in the experience of others on the mystery of the decel buzz.

Cheers
Alex

[slomo]

Does the noise alter or go away if you hold the shifter and rotate gently clockwise.?

[Mr Morlock]

You can well and truly overthink these things and look for unlikely causes. Just try a few things- if its an oil change that's simple.