Deceleration Rattling/Grinding sound
 

First, I know this has been beat to death on miata.net, but I wanted to ask before I go throwing money at the problem. I will do my best to be as detailed and straight forward as possible in order to keep things simple.

I recently upgraded the clutch on my 2000 LS with 87k on the clock, to a Supermiata sport organic clutch kit. While doing this, I also installed a 6 speed that ended up being bad. With the 5 speed back in the car, I have the well documented deceleration noise (metallic rattling/grinding) between 3-4k in 2nd-4th gear. It can be heard slightly when accelerating as well. Im having trouble pinpointing what area of the car it is coming from. My wife thinks it is coming from the rear of the car, and I think she might be right. It should also be noted that I have a full aftermarket exhaust and thus no heat shields on the piping itself.

Here is the order in which I have done things:

1. 5 speed out. Supermiata clutch kit + resurfaced stock flywheel and 6 speed in.
2. 6 speed out due to excessive bearing noise. Checked for damage to clutch assembly and internal transmission damage, none found. 6 speed back in.
3. 6 speed out due to horrible transmission noise in neutral. 5 speed back with Redline MT 90. PPF raised higher than before with jack, but not to spec.
4. PPF aligned to spec, exhaust adjusted due to it contacting the subframe on turns and rough roads.
5. Diff gear oil changed.
6. PPF unaligned to natural droop (this actually puts the shifter when it was before the transmission was pulled the first time).
7. PPF re-aligned to spec.
8. PPF re-aligned.

My current guess is a failing diff, but like I said, I wanted to see what you all think before I go throwing money at the problem by having my 4.3 rebuilt or sourcing a 4.1.

The 5 speed can make some interesting noises. Do you have a torn shift boot that might be transmitting more NVH than before?

My 5 speed has always made noise like this, not really sure that it's something that can be fixed. My plan is to eventually turbo the car, upgrade the rods, and push the boost til the 5 speed fails, at which point I will swap in a 6 speed. Should solve that specific noise issue.

My shift boot isn't in the best shape, and it is missing the plastic retaining ring. To be honest though, I can't imagine that any manner of insulation would block this sound.

Its amazing how widespread of an issue this is. There has to be a direct cause. Im planning to turbo my car as well, but I want to get all of the driveline and maintenance items sorted out first.

If this is a B6 clutch kit and light flywheel, it's going to be pretty noisy. Not enough mass to dampen the vibrations that are always there. The BP kit has enough extra mass that noise is almost nil. Direct trade off in NVH for quicker revving.

Something making a sound =/= an issue. We've been trained to believe this though, reading through all the threads of engine noise when DI became popular is great. "is this knock" "my new motor is ticking what's wrong" etc etc.

Its a BP kit with a stock flywheel.

I generally understand this. Its the fact that the alarming metal on metal noise appeared after I changed something that concerns me. Also, the fact that I can't pinpoint where it is coming from.

Did you set your PPF height correctly when you installed the trans that’s currently in the car?

Not the first time I installed this current transmission, but I set it to spec later on. No noticeable change in noise when I did so. I will say my frame rails are pretty beat up, so it was difficult to find a place to measure from.

Put car on lift or securely on 4 jack stands and get beneath it with a mechanic's stethoscope and attempt to recreate the noise. Pinpoint it and stop guessing.

I think the sound is the result of it being under load. The load being the drag of the engine against wheel speed. Kind of hard to re-create on a lift. I'll also admit driving the wheels with the car in the air makes me uncomfortable as is, being under it... no thank you.

You can take the rear wheels off for less of a risk of the car falling. Use 2 jackstands under the jack pinch welds, 2 jacks under the control arms, and a jack under the diff. 3 points of failure, no wheels to knock/hit anything, and you should be fine. Would also help determine if taking weight off/the wheels off has anything to do with the noise.

Come on. If De Niro can do it in Cape Fear, so can you.

Remove the wheels if you wish. Make sure it is secure, of course. I always bump the car to ensure stability as soon as I set it on stands. Don't put stands under moveable parts.

If a load is required, have your assistant add some e-brake drag one click at a time while holding the necessary rpm. This isn't rocket science.

Mine has made this noise on decel for the last 20k miles with no changes. Though I have the 13lb FM1 happy meal. I am electing to ignore it, just like the straight cut gear sounds it makes.

I might give this a go this weekend. I have a set of quick jacks, so thats probably better than jack stands, at least.

I'm going to try and post a video later. Im curious to see if we have the same noise. Im coming to the realization that I probably need to just live with it. If the diff goes, then i'll just get a 4.1, if the trans goes, then i'll be glad I only have a 5 speed lol.

I haven't had a chance to put the car up on the lift yet, but I found this thread on another forum and wanted some more insight. https://www.mx5nutz.com/forum/index....owtopic=101863

Basically, one user asks about the decel noise and another user chimes in with the idea the pinion nut might have backed out and the pinion is coming in contact with the differential itself. Apparently this actually happened to this user, and he had pictures to prove it. I thought this was interesting and might actually make sense given my symptoms.

On page 03-14-10 of the factory service manual, it discusses the procedure for tightening the pinion lock nut in terms of pre-load adjustment and tightening torque. I am a bit confused and to be honest, and no nothing about setting up a diff. Can I just take a torque wrench and set it to the tightening torque range to ensure or is there more to this procedure, as the manual makes it seem.