Why does my Miata buzz? Help!

I need your guys' help before I go insane here. Recently my '90 Miata has developed a hideous heatshield buzz/rattle noise. IT started about a month ago, was getting ready to go on a road trip and was driving to my friend's house to pick him up, then after he got in it stopped. Then a few weeks later it started doing it again and I have been unable to figure it out.

The noise: As I said, it's a very loud, prominent heatshield buzz/rattle noise. I can hear it over my damn loud exhaust, so I'm not chasing something minor. Can hear it easily with the top up. It's still there with the top down, though not as audible. Not sure if it's drowned out by all the other noise you are hearing with the top down, or if putting the top down changes the noise. Seems to come from inside the cabin behind the driver but I know how noise can transmit, so I wouldn't put too much stock in that. Not sure if it's audible outside the vehicle, bit difficult to check.

When it does it: It's on accel or decel as I pass the 3500-3700rpm range. Can duplicate it reliably. Does it every time. Doesn't seem to do it while it's not moving. Does not matter what gear it is in. (Haven't checked it in Reverse, but I assume it will. I have no intentions of checking that.)

What I've tried: Cleaned everything out of the trunk, zero effect. Checked the bolts on the trunk closeout panel, they were tight, but removed the whole panel just to be sure, zero effect. Checked the bolts/screws/fasteners on package tray panels, a few were loose but tightening all of them had zero effect. Checked the heat shields underneath, all fasteners present and tight. Removed the heat shields and checked to make sure there was no gravel/rocks wedged up there and brushed them off. Noise still present. Remembered some change sliding out of the cupholder area and down under the carpet, so removed the seats and console, peeled the carpet up and cleaned everything out from under it, noise still present.

Anybody encountered anything like this, or have any suggestions? It is seriously driving me insane and I cringe anytime I get near that RPM range because I know that damn awful buzz is coming.

Motor or transmission/diff mounts?

Do you have any bracing on the car?

It could be a resonate frequency within the chassis causing the doors or some piece of trim to vibrate.

Heat shield over the exhaust manifold buzzes at about that frequency when it's cracked. Second option is the cat heat shield.

In reply to Keith Tanner :

It has a header, so the stock manifold heat shield is no longer there. And it no longer has a catalytic converter anymore either, has an ISR resonated test pipe. The two remaining undercar heat shields both appear to be intact, no tears or cracks.

My silver nb had the tin under the package tray capet rattle the bolts loose.

In reply to Dusterbd13-michael :

Yup, checked all those. They were my first thought. There were one or two loose, but tightening them all didn't change the noise at all

Does the na have a shield unde the tank and above the muffler?

Does it every time. Doesn't seem to do it while it's not moving. Does not matter what gear it is in.

That's weird. It's gotta be moving, but it's not speed sensitive? What if you put the car in neutral and free rev? What if you get the rear wheels off the ground? I'm starting to think we should be looking inside the bellhousing. 

Does the noise come from a specific area that you've been able to identify?

To much ethanol, not enough methanol?

I do have Keiths old Mazda NVH book I could send your way if you think it could help. 

I had a nasty buzz that came from one of the torsen bars that hold the trunk open unclip from the mount.  Did you already check those?

WonkoTheSane said:

I had a nasty buzz that came from one of the torsen bars that hold the trunk open unclip from the mount.  Did you already check those?

I have not checked that, and that would seem to match the area in hearing it from. Would make sense it went away on our road trip because we had the trunk packed right full

Keith Tanner said:

Does it every time. Doesn't seem to do it while it's not moving. Does not matter what gear it is in.

That's weird. It's gotta be moving, but it's not speed sensitive? What if you put the car in neutral and free rev? What if you get the rear wheels off the ground? I'm starting to think we should be looking inside the bellhousing. 

Don't say that, I've had the trans out enough over the ownership of the car. As it sits, its a low-ish miles 6-speed, with a brand-new (under 6k miles) FM flywheel, clutch and pressure plate, all torqued and loc-tited. Torqued and loc-tited the input shaft plate bolts too, after the incident that killed the 5-speed that was in it.

EDIT: Tried this morning, and it will make the noise when in neutral and free-revving it. So basically, any time I hit the 3300-3700rpm range, it makes the noise.

Gonna sound weird but check the rear brakes for play (grab the parking brake cable and wiggle). Easy to test by brushing the brake pedal when the noise happens and seeing if it changes or goes away.

I had a very similar noise that I chased.

Robbie said:

Gonna sound weird but check the rear brakes for play (grab the parking brake cable and wiggle). Easy to test by brushing the brake pedal when the noise happens and seeing if it changes or goes away.

I had a very similar noise that I chased.

Tried it. Was going down the road and both pulled up gently on the e-brake handle and gently left-foot braked and the noise was still there, unchanged

In reply to WonkoTheSane :

Checked the torsion bars on the trunk this morning. All nice and clipped in like they are supposed to be

What about the shield over the fuel filler neck in the trunk?

joey48442 said:

What about the shield over the fuel filler neck in the trunk?

Checked that early on, because mine has a bolt broke off in the floor, so one tab isn't fastened. All the other fasteners were tight, but I removed the closeout panel entirely and have been driving around without it. Had no effect.

NickD said:

Keith Tanner said:

Does it every time. Doesn't seem to do it while it's not moving. Does not matter what gear it is in.

That's weird. It's gotta be moving, but it's not speed sensitive? What if you put the car in neutral and free rev? What if you get the rear wheels off the ground? I'm starting to think we should be looking inside the bellhousing. 

Don't say that, I've had the trans out enough over the ownership of the car. As it sits, its a low-ish miles 6-speed, with a brand-new (under 6k miles) FM flywheel, clutch and pressure plate, all torqued and loc-tited. Torqued and loc-tited the input shaft plate bolts too, after the incident that killed the 5-speed that was in it.

EDIT: Tried this morning, and it will make the noise when in neutral and free-revving it. So basically, any time I hit the 3300-3700rpm range, it makes the noise.

What about with the clutch disengaged?

if you can do it free revving, then put it up on jack stands and enlist a friend. 

Keith Tanner said:

NickD said:

Keith Tanner said:

Does it every time. Doesn't seem to do it while it's not moving. Does not matter what gear it is in.

That's weird. It's gotta be moving, but it's not speed sensitive? What if you put the car in neutral and free rev? What if you get the rear wheels off the ground? I'm starting to think we should be looking inside the bellhousing. 

Don't say that, I've had the trans out enough over the ownership of the car. As it sits, its a low-ish miles 6-speed, with a brand-new (under 6k miles) FM flywheel, clutch and pressure plate, all torqued and loc-tited. Torqued and loc-tited the input shaft plate bolts too, after the incident that killed the 5-speed that was in it.

EDIT: Tried this morning, and it will make the noise when in neutral and free-revving it. So basically, any time I hit the 3300-3700rpm range, it makes the noise.

What about with the clutch disengaged?

if you can do it free revving, then put it up on jack stands and enlist a friend. 

Haven't had time to put it on a lift ior jackstands, but on lunch I went out and checked to see if it made the noise with the clutch disengaged. Clutch in or out has no effect.

You said it stopped when someone was in the passenger seat. 

Have you tried installing a passenger again? If that stops the noise, it has to be something associated with the seat, the mass in the seat changing the shape of the floor, where the passenger put their feet on the floorboards, or what they were leaning against (particularly if you were doing some spirited driving and they were bracing)...

maybe...

NickD said:

In reply to WonkoTheSane :

Checked the torsion bars on the trunk this morning. All nice and clipped in like they are supposed to be

Ah drat, I was hoping for an easy one :) 

Robbie said:

Gonna sound weird but check the rear brakes for play (grab the parking brake cable and wiggle). Easy to test by brushing the brake pedal when the noise happens and seeing if it changes or goes away.

I had a very similar noise that I chased.

Interesting. I thought of you when I first read this thread. That was the brakes?

Could it be the ECU cover in the passenger side footwell?

cmcgregor said:

Could it be the ECU cover in the passenger side footwell?

Not on the vehicle. I removed it when I did the MegaSquirt install last year, and it had some gnarly surface rust that I wanted to sandblast and primer, as well as needing the studs in the floor bracket replaced because they broke off, and I never got around to doing it. A year later, still no ECU cover. That reminds me, I really should get that back in.