What can cause an exhaust drone?

Gazbo · Postby **Gazbo** » Wed Jul 16, 2008 8:52 am

My NA6 with original & standard exhaust has recently developed a loud-ish drone noticeable when cruising over 100km/h and on over-run (lifting off).

Could this be caused by a damaged catalytic convertor?

The car was runing on 3 cylinders a few weeks back (water in spark plug well) so the cat has had a dose of unburnt fuel.

Steampunk · Postby **Steampunk** » Wed Jul 16, 2008 9:34 am

loose nuts maybe? or perhaps a hole that has developed somewhere

bruce · Postby **bruce** » Wed Jul 16, 2008 11:57 am

Unlikely to be the cat. More than likely a hole or a collapsed baffle in the muffler. Get the local exhaust shop to throw it onto their hoist and have a squizz.

Gazbo · Postby **Gazbo** » Wed Jul 16, 2008 2:32 pm

bruce wrote:Unlikely to be the cat. More than likely a hole or a collapsed baffle in the muffler. Get the local exhaust shop to throw it onto their hoist and have a squizz.

Will do... I had a quick look under there myself but couldn't see anything obvious. Nothing rattles when I tap the muffler or cat with a mallet.

AJ · Postby AJ » Wed Jul 16, 2008 4:26 pm

X2 on the collapsed baffle in the muffler

Gazbo · Postby **Gazbo** » Tue Jul 29, 2008 7:48 pm

Well, I tried fitting a new muffler (cat-back) and still I have this droning noise, especially noticable when decelerating.

If the engine is revved whilst stationary, you can hear the noise as the revs drop back to idle. That discounts all the drivetrain parts like differential, driveshaft, universal joints etc.

Seems to be related to a noise discussed at length in a miata.net thread, where most people call it the "transmission deceleration noise". The cause was attributed to everything from leaky gearbox seals to misaligned PPF.

Probably unrelated, but I have an oil leak from the clutch fork boot (NOT the slave cylinder) - how could that occur? Didn't think there was any oil behind that boot.

I suppose it still could be an exhaust leak (headers/downpipe/gasket), but I (or a muffler shop) couldn't find anything.

Any ideas?

AJ · Postby AJ » Tue Jul 29, 2008 8:04 pm

Gazbo wrote:Probably unrelated, but I have an oil leak from the clutch fork boot (NOT the slave cylinder) - how could that occur? Didn't think there was any oil behind that boot.
Any ideas?

that oil will be coming from the rear main seal of the engine Gazbo, bit of a pain, & won't be doing your clutch any good, the box has to come out to replace it.

Gazbo · Postby **Gazbo** » Tue Jul 29, 2008 11:15 pm

After lots of research on miata.net, seems many people have this problem. Best described as a "deceleration growl", happens both in gears whilst moving and revving engine in neutral whilst stationary.

Unless I'm very lucky or persistent, I'll probably just have to live with it.