Metalic grinding noise from rear when no accelerator

[Oni]

well basically topic, i was comming down a hill and went from 5th to 4th and when i didnt accelerate i hear this metalic kinda noise sort of grinding fmro the rear, or i think it was from the rear, on the rest of my 5min trip home i tried swapping to each gear and then cruising without the accelerator on and it did the same Any ideas? ill try get more of an idea about the noise tomorrow and edit this secion to try aid in some help. could it possibly be a rock that i can only hear when not accelerating ? i hope its nothing major thanks in advance!


Oni

[Fatty]

could be your rear brakes , something to do with the handbrake being stuck on. i think the slider pin thingos on the calipers can get a bit rusty and stick... hopefully someone else can provide the actual details coz i remember it has been discussed here.


i had a grinding metallic noise coming from the front recently, but that was coz one of my brake pads was in backwards

[Oni]

i see thanks! i checked basic things like handbreak off then put it on and off again but the pads might have still stuck. its a '60 NA6 is that might matter . thanks for the fast reply i might take a wheel off and see if i can see any rust.

[StanTheMan]

a 47 yo car?!?

hopefully its what fatty said.....lots of other more expensive things like bearings ,Diff & gearbox...

[Oni]

gee i mean '90 lol, yeah i really hope thats it. havent heard a thing from it untill today

[Oni]

OK well i drove to work again today and did some tests. the noise doesnt happen in 1st and 5th. it happens when in 2,3,4 and between 2.2k revs and 3 and also when im not accelerating. The noise sounds a little like when u try to go back a gear when going to fast, or like when u dont pick a gear properly. Its also started to make it when i accelerate a bit harder than normal. Also i didnt get to check the breaks but wouldnt it make the noise all the time if they were stuck on? probibly sould be worse when accelerating if they were. Thanks for any help. i hope its not my gearbox, could possibly a different spec oil help?. the sound also reminds me of metal vibrating/shaking. Thanks for any help

[Richee]

Hi Oni

A number of MX5s do this - this issue has been discussed before and is vibration from the gearbox due to wear, I searched quickly but couldnt find it.

Jules knows all about this - Jules where are you??

Nothing you can do about it, perhaps change the gearbox but it could still be there.

Richard

[manga_blue]

Resonance point in the exchaust? Does it rattle when you shake it or bash it?

[adamjp]

Has your diff suddenly sprung a slight leak?

If you are running the stock diff it is possible that the centre of the diff has cracked and you are only a few kms from a complete diff destruction.

My diff started leaking, so I topped it up and wondered how much some new diff seals were. Slight grinding noise, but didn't think much of it. I drove the car to my parents place down the coast and then just rolling to a stop at an intersection, CRACK! the diff let go. I was lucky to be able to get the car off the road before the rear wheels locked up completely.

All up around 250km (including some pretty spirited driving) from leak to failure. It started leaking because the diff centre had cracked and the output stub axle shaft was not running true anymore. The cracks are fairly apparent in this picture taken after I pulled it out. The hole in the cast iron centre is where one of the output stub axles comes out.


Then again your problem may be simply related to the brakes, including a rock between disk and shield.

[Oni]

@ manga_blue
When i kick it, it moves about 2cm in all directions but theres not sort of rattling sound sort of a hollow gong that i presume was from the force of my boot.

@ Adam
How can i check this? i dont really know much but i really dont want my car to break. I would presume its a totally stock diff. if i stuck my head under the car ( its been parked for a few hours now since im at work) i should see some liquid on the ground if its cracked? or would i need to get it raised and have a look underneith?

Thanks for the replies all.

[jules]

Hi Oni

A number of MX5s do this - this issue has been discussed before and is vibration from the gearbox due to wear, I searched quickly but couldnt find it.

Jules knows all about this - Jules where are you??

Nothing you can do about it, perhaps change the gearbox but it could still be there.

Richard

This doesn't sound like the issue I had. Mine was a rattling on deceleration between 3000-3500rpm, that was caused by wear in the gearbox casing where the shift rods sit. It required a rebuild (done under warranty by Eurocars).

I originally thought it was heat shields as it was a similar sound.

Jules

[adamjp]

should see some liquid on the ground if its cracked

Absolutely. The scenario that I have outlined is probably the worst case, but your description is reminiscent of my diff failure.

1. Does the sound change according to road speed or engine speed? This will localise the car sub-system that is faulty. Road Speed is everything from the gearbox to the wheels. Engine speed is everything from the clutch to the engine (including the exhaust).

2. If road speed, does it change when you corner, brake, accelerate or cruise?

3. If engine speed, does it change when you rev the car out of gear (standing with bonnet open, head in engine bay, using direct hand-throttle connection)? Or does it change according to engine load - 4th gear when you should be in 2nd, on the overrun (using the engine to brake when shifting down)?

4. If the sound changes when you corner, wheel bearings are the first likely item. You will tell which side because when that side gets light in a corner, it will stop making noise. Other causes could be something knocking/touching the drivetrain (like your exhaust due to a broken rubber).

5. Acceleration - gearbox/diff. Sometimes clutch.

6. Deceleration, usually brakes.

7. On the overrun, exhaust shielding or clutch.

8. Under load, engine - possibly pre-detonation.

9. On the cruise - NFI.

Work through these and get back to us.

I've never heard of the shift linkages rattling as suggested, but that does not mean it couldn't happen.

[jules]

Some boffin Japanese engineer from Mazda Japan was out here and was at Eurocars at the time. They couldn't work out the noise, but he found it, and after replacing the gearbox casing the sound went permanently.

I have never heard of it before, or since.

Jules

[bensale]

I have the same bloody rattle, it seemed that every MX5 I looked at had it. If anyone knows how to fix it please tell, the gearbox casing sounds interesting... Mine only occurs after 15 minutes of driving once the car is warmed up and at around 3000 RPM on deceleration when engine breaking.

My diff looks fine.

I've recently been reading this thread http://forum.miata.net/vb/showthread.ph ... ge=1&pp=25
Have fun reading, theres a lot of info

Its apparently very common and in a some cases people have had the noise since new, 180,000+ miles later no issues due to it.

The whole thing really confuses me.....Having read the post on this tread and the US thread you cant be sure its the same noise but with my limited knowledge possible fixes/problems could include....

The diff's connection with the driveshaft, fixed through putting a 1.8 diff in as these have a damper, where the 1.6 diff doesn't.

Incorrect mounting of the PPF, the brace that conects the gearbox to the diff.

I'm going to check this next time I drive, if you press the clutch in 1/2 an inch during engine braking and the noise stops it could be the clutch adjustment.

Apparently the noise is often caused after changing the clutch.

Interestingly my Dad used to have a Triumph TR4 that made exactly the same noise, so maybe its just down the the gearbox design. If there isnt an easy solution I think i'll live with it as it isn't really that big a worry, that being said I would love to not have it.

[bensale]

Jules, I though mine also sounded like the heat shields,

that was caused by wear in the gearbox casing where the shift rods sit. It required a rebuild

They couldn't work out the noise, but he found it, and after replacing the gearbox casing the sound went permanently.

Not a cheap fix I asume, and fromt the variety of problems no guarentee of one size fits all fix....