Mystery rear end noise... Ooo, err...

Post here if you require help or have information that may assist others with their MX-5.

Mystery rear end noise... Ooo, err...

Postby bmv6197 » Fri Sep 18, 2015 2:48 pm

Hi folks,

I'm looking for a bit of advice on the cause of a fairly low noise from the rear end of my Mk1. I've noticed it over the past few months, before which I think it was present but fell under the category of "general road noise", but the more I listen to it, the more I think the car's trying to tell me something!

The best way to describe it is it's like the sound of blowing over the top of a glass bottle - not that different to general tyre noise. It's not overly loud but more audible with the roof closed. It is a constant pitch, so doesn't change with road speed or engine speed, and is present from about 15-20mph right up to 70 when it perhaps seems quieter but probably just because all the other road/engine/wind noises are louder.

So far, I've followed a process of diagnosis by elimination, but so far without success:

- Tyre noise: checked tyre pressures - all fine; swapped rear tyres/wheels for my winter ones - result: no change in the noise
- Brake drag: cleaned up disk edges so no rusty surfaces to scrape; checked brake temperatures after driving - no evidence of pads rubbing; tried driving with handbrake on - result: no change in the noise
- Worn wheel bearing: tried "weaving" and checked sound for changes on left/right cornering - result: no change in the noise; put the car up on a ramp - all 4 wheels fine with no sign of noise or play in any of them; as I had a spare new rear wheel bearing I got the left rear one swapped out anyway as that was the side I "thought" the sound was coming from - result: no change in the noise
- Noisy diff: drained and replaced diff fluid with Redline 75w90 and checked old fluid condition and searched for any metal bits, but it all looked perfect - result: no change in the noise

The car is a UK spec 95 NA 1.8iS with the standard Torsen LSD. It's only done 50k miles, had a full Mazda service history when I got it 9 years ago and has had an even more extensive service history since I've owned it.

I'm at a bit of a loss as to what it could be. I'm not worried about any imminent failure as everything seems tight and solid, but it is now really annoying me that I haven't been able to identify it. I need to drive the car down south for a few days next week so would really like to solve the mystery.

Any thoughts or ideas would be appreciated.

Cheers,
Brad.
1995 Mazda MX5 1.8iS (BRG)
1996 Eunos Roadster VR Limited (Combination B) [Now sold]
bmv6197
User avatar
 
Posts: 204
Joined: Tue Jul 12, 2005 12:39 pm
Location: Aberdeen

Re: Mystery rear end noise... Ooo, err...

Postby drumtochty » Fri Sep 18, 2015 3:06 pm

It could be the little flaps at the bottom of the cabin drain tubes that should auto close when no water is draining that have been stuck open or missing.

The can cause a constant noise not very noisy but enough to annoy.
99 Mk2 1.8 IS Silver
08 Mk3 2.0 RC Sport Copper Red
drumtochty
User avatar
OC Member
Eddie Cairns
MX-5 OC Club Member
 
Posts: 2584
Joined: Thu Jun 18, 2009 1:42 pm
Location: Drumtochty Glen, Auchenblae

Re: Mystery rear end noise... Ooo, err...

Postby bmv6197 » Fri Sep 18, 2015 3:08 pm

I should add that I've done a lot of searching the various MX5 and Miata forums and can't find any similar issues. Diff problems seem to be far more distinctive in sound and vary with acceleration/deceleration/coasting and more common on the VLSDs than the Torsen, and seem to affect cars with greater than 80k miles. Wheel bearings have the familiar variation when turning left or right and vary in pitch with road speed (plus I've replaced one wheel bearing now anyway). And I can't find any cases where the opposite end of the drive shaft to the wheel bearing (i.e. the differential bearings) has needed replacement...

B.
1995 Mazda MX5 1.8iS (BRG)
1996 Eunos Roadster VR Limited (Combination B) [Now sold]
bmv6197
User avatar
 
Posts: 204
Joined: Tue Jul 12, 2005 12:39 pm
Location: Aberdeen

Re: Mystery rear end noise... Ooo, err...

Postby bmv6197 » Fri Sep 18, 2015 3:10 pm

drumtochty wrote:It could be the little flaps at the bottom of the cabin drain tubes that should auto close when no water is draining that have been stuck open or missing.

The can cause a constant noise not very noisy but enough to annoy.


Thanks Eddie - it can't hurt to try. I'll pop out, tape 'em up and try another road test!

Brad.
1995 Mazda MX5 1.8iS (BRG)
1996 Eunos Roadster VR Limited (Combination B) [Now sold]
bmv6197
User avatar
 
Posts: 204
Joined: Tue Jul 12, 2005 12:39 pm
Location: Aberdeen

Re: Mystery rear end noise... Ooo, err...

Postby bmv6197 » Fri Sep 18, 2015 4:13 pm

Good suggestion Eddie, but just back from a road test and the noise persists...

I did find a quiet lane and can confirm that at 15+ mph in reverse the noise is present too - under power or coasting... So it's unlikely to be any sort of wind noise as the aerodynamics of the car are (unlike a Boxster) quite different in reverse ;-)

B.
1995 Mazda MX5 1.8iS (BRG)
1996 Eunos Roadster VR Limited (Combination B) [Now sold]
bmv6197
User avatar
 
Posts: 204
Joined: Tue Jul 12, 2005 12:39 pm
Location: Aberdeen

Re: Mystery rear end noise... Ooo, err...

Postby 71NotOut » Fri Sep 18, 2015 5:01 pm

Any missing grommets in the boot floor or something like that?

What kind of wheels do you have on it? Maybe try other wheels if you can borrow a set?

Failing that, turn the stereo up :D
Previously in a 2003 Montana, now in a 1991 V-Special...
71NotOut
User avatar
OC Member
Steven Hedley
MX-5 OC Club Member
 
Posts: 408
Joined: Tue Apr 20, 2010 1:29 pm
Location: Bilston

Re: Mystery rear end noise... Ooo, err...

Postby joeytalent » Fri Sep 18, 2015 5:01 pm

Can you hear it outside the car? Perhaps you can get a glamorous assistant to slowly drive past to try and narrow it down.

Sent from my ASUS_Z00AD using Tapatalk
joeytalent
 
Posts: 424
Joined: Mon Jan 31, 2011 4:36 pm

Re: Mystery rear end noise... Ooo, err...

Postby bmv6197 » Fri Sep 18, 2015 5:17 pm

71NotOut wrote:Any missing grommets in the boot floor or something like that?

What kind of wheels do you have on it? Maybe try other wheels if you can borrow a set?

Failing that, turn the stereo up :D


No plugs, grommets, blanks or other trim missing, and nothing extra added! My winter tyres are on different alloys (still Mazda ones though) and the sound occurs using either set... Thanks for the ideas though!

B.
1995 Mazda MX5 1.8iS (BRG)
1996 Eunos Roadster VR Limited (Combination B) [Now sold]
bmv6197
User avatar
 
Posts: 204
Joined: Tue Jul 12, 2005 12:39 pm
Location: Aberdeen

Re: Mystery rear end noise... Ooo, err...

Postby bmv6197 » Fri Sep 18, 2015 5:22 pm

joeytalent wrote:Can you hear it outside the car? Perhaps you can get a glamorous assistant to slowly drive past to try and narrow it down.

Sent from my ASUS_Z00AD using Tapatalk


Thanks Joey - I couldn't find a glamorous assistant but went with a less glamorous one, who could hear the noise from the passenger seat but it's not loud enough to hear above external engine and tyre noise if I leave them on the side of the road and just do a "drive-by"!

I'm starting to wonder if it's one of the differential bearings at the end of one of the driveshafts (although surely the new fancy diff fluid would lubricate them and have changed the sound somehow)... But I'm just guessing now as all the logical answers haven't worked!

Cheers,
B.
1995 Mazda MX5 1.8iS (BRG)
1996 Eunos Roadster VR Limited (Combination B) [Now sold]
bmv6197
User avatar
 
Posts: 204
Joined: Tue Jul 12, 2005 12:39 pm
Location: Aberdeen

Re: Mystery rear end noise... Ooo, err...

Postby drumtochty » Fri Sep 18, 2015 6:26 pm

No the Redline is a bit near the runny side.

I'm also veering to the diff, try some 140 grade GL5 oil.

Not convinced it will make that much difference, many years ago back street traders would put in sawdust in a diff or gearbox to get a car sold with the noise not apparent until a week or so later.
99 Mk2 1.8 IS Silver
08 Mk3 2.0 RC Sport Copper Red
drumtochty
User avatar
OC Member
Eddie Cairns
MX-5 OC Club Member
 
Posts: 2584
Joined: Thu Jun 18, 2009 1:42 pm
Location: Drumtochty Glen, Auchenblae


Return to Technical

Who is online

Users browsing this forum: No registered users and 5 guests

cron