Minor metallic noise from the rear under acceleration?

[Gizmo!]

I've not had my MX-5 long, and it's my first, so I'm hoping for a little bit of your experience here in establishing whether a 'noise' I've heard is worth being worried about and getting someone with spanners (possibly me) to investigate.

In short - Saturday afternoon I was out for a drive with the roof up (rain was looking threatening). Under acceleration - and not at other times - there's a noise, quiet but distinct, from the rear of the car. It sounds like two metal surfaces running against one another, not in a scraping kind of way, just a mild skimming. You can't hear it with the roof down - I guess the dampening of the roof on the rear deck cancels it out, let alone the wind noise.

I've discounted a rubbing brake or binding handbrake, because I don't see why they would make a noise only under acceleration. Not a wheel bearing - it does it in straight lines and around corners in both directions. Could it be the differential? Under load I guessing you're putting power through the surfaces.

Any thoughts?

[Stickers]

My MK1 gives out a similar noise @ approx 3000rpm - it's coming from the heat shield, vibrating mid way along the exhaust.......interestingly a friend thought it sounded like a supercharger, although it doesn't sound like any supercharger I've ever heard!

[tuttle]

mine also vibratesfrom this area at 3krevs. Your noise does sound like brakes binding though.

[bluetone]

Remember that under acceleration the geometry changes slightly as well as the rear squats down/the nose rises. Maybe it is this action that is causing two surfaces to come into contact?

[MX-5 Lazza]

Also remember that a noise that sounds like it is coming from the back might well actually be coming from under the car somewhere (gearbox, clutch, drive-shaft, dif, exhaust, heat-shields etc.) or even from the front. This is because the hood drain-tubes transmit sound from under the car into the cabin behind your head. This is also why a lot of these noises are only heard with the top up

[phil_cardiff]

My mk2 also makes a raspy rattling noise only under accelaration. Seems most evident in 4th gear from low revs. I've always though it sounds like a manifold or exhaust rattling. Considered the lsd too. Either way I plan on asking when she's next serviced but hope it's not serious.

[Gizmo!]

Hmmm.
Ah well, it'll wait until it gets serviced.

[big_lad]

phil_cardiff said:

My mk2 also makes a raspy rattling noise only under accelaration. Seems most evident in 4th gear from low revs. I've always though it sounds like a manifold or exhaust rattling. Considered the lsd too. Either way I plan on asking when she's next serviced but hope it's not serious.

Hmmmm, mine sounds very similar to that; hope it's NOT the LSD as that sounds pricey! Will you post on here when you get that service..?

[MX-5 Lazza]

If it's 4th gear ONLY and at between 1.5-2krpm and again around 4krpm then it's just the 2nd gear selector chattering in the gearbox and is absolutely nothing to worry about, just a poor gearbox design.

[phil_cardiff]

I think it does it in 3rd too, but it's not as noticable. I will investigate.

Big lad - it's due a service in July/August so not long to wait.

Just to add I'll be changing the diff oil at this service too so should find out the state of the lsd.

Edited by phil_cardiff on Thursday 11th June 18:46

[phil_cardiff]

MX-5 Lazza said:

If it's 4th gear ONLY and at between 1.5-2krpm and again around 4krpm then it's just the 2nd gear selector chattering in the gearbox and is absolutely nothing to worry about, just a poor gearbox design.

Tested it today. Definitely more pronounced in 4th gear. Between 1.5 and 2krpm and again at about 3krpm-4.5krpm. Sounds like it's the 2nd gear selector

[MX-5 Lazza]

Loads of them have the same noise. It doesn't cause a problem other than noise irritation so if the noise isn't bothering you then just ignore it.

[big_lad]

MX-5 Lazza said:

Loads of them have the same noise. It doesn't cause a problem other than noise irritation so if the noise isn't bothering you then just ignore it.

Cool cool, that's a relief to me at least Thanks Lazza!