funny rumble

[luket]

Hey all, I'm recently became aware of a new sound, spent a good deal of time looking through past post but haven't found anything quite like it. I would appreciate any theories to check.

Spider 2000
32/36 dfev and single pane manifold
Port matched and polished head
Stock exhaust

I first pinpointed the prevalence of this a few weeks ago, at night after driving several hundred miles cross country. Since then I notice it more, but it may be psychological. There has always been some vibration at specific rpms, but its starting to bug me and I want to smooth it out.

There is a low audible rumble and vibration in the shifter at low/mid rpm. I believe it is present at all rpm below 2200, but becomes noticeable around 1700-1800 rpm. The noise increases with rpm and whe the engine passes 2200 the rumble completely stops and it is a smooth as butter all the way up to 5000. When things warm up more, or its a hot day, I notice the rumble sticks around until 2500.

I cleaned up the plugs, gapped them, and added fresh dielectric grease today with no change to the sound.

Planning to check the valve clearance tomorrow but the sound doesn't fit that high pitched slap I'm used to with big clearances.

Maybe the low speed jet / mixture ?

Hoping it has nothing the do with the bottom end.

Any help is greatly appreciated

[RoyBatty]

check the flex coupling at the transmission output shaft.
Also known as the giubo. Pronounced "jewbo".

[luket]

Will check but it does it in neutral when revving without trans engaged

[luket]

I should also note that the no. 4 plug had a bit of white on it, indicating a small leak. Anyone think this could cause the symptom?

[RoyBatty]

Does the noise at idle change if you depress and release the clutch?

[luket]

No, clutch doesn't affect it

[81SPIDERMATT]

Does the noise at idle change if you depress and release the clutch?

may I ask what the answer would have been had original poster said yes to that question

[focodave]

Will check but it does it in neutral when revving without trans engaged

This is a long shot, but have you considered the motor mounts possibly being worn out?
That could cause a rumble. Perhaps the engine is sagging and it is rumbling against the cross member.
At higher RPM and ground speed, it could be less noticeable.

[luket]

I will check the motor mounts but they have looked pretty good. I think its a good theory.

I did do a check with the clutch in and out. With the clutch out the vibration cuts out at maybe 100-200 rpm lower. This would likely be due to a bit more mass participating in the harmonic with the clutch spinning, lowering the natural freq. Ppl may disagree but I've got a good deal of structural dynamics analysis experience, so I'm pretty confident that's what's going on. I think something somewhere in the system has a bit of slop. I've seen similar phenomenon in aerospace with a harmonic cutting out instantanioualy when the amplitude of a response gets large enough to close a gap, changing the support points of a component and instantaneously changing the natural frequencies of the system. We call it a nonlinear harmonic system. Okay. Sorry guys, I digress.

[DanD]

Did you check your intake manifold bolts ? You seem to be implying that you have a lean condition in one cylinder. Have you done a compression check ?

Unbalanced rotating assemblies can have weird harmonic imbalances at certain RPMs, and be fine at others. but it doesn't come and go. One of the strongest Fiat engines I ever drove was a Waring Blender in fifth at 65-70 MPH, but smoothed out wonderfully at 75-90. That engine went on forever.

I don't know nothin about building no engines like some of the guys here, but as a general rule of thumb, if I have an engine apart, I will balance the pistons and rods, and send the crank and flywheel out to be balanced. Especially if I am mixing and matching stuff from different engines.

Most common imbalances on a Fiat are bad U joints, bad center drive shaft bearing, and front and rear drive shafts being 180 degrees out from a rebuild. But none of those match your symptoms.

[focodave]

I will check the motor mounts but they have looked pretty good. I think its a good theory.

I did do a check with the clutch in and out. With the clutch out the vibration cuts out at maybe 100-200 rpm lower. This would likely be due to a bit more mass participating in the harmonic with the clutch spinning, lowering the natural freq. Ppl may disagree but I've got a good deal of structural dynamics analysis experience, so I'm pretty confident that's what's going on. I think something somewhere in the system has a bit of slop. I've seen similar phenomenon in aerospace with a harmonic cutting out instantanioualy when the amplitude of a response gets large enough to close a gap, changing the support points of a component and instantaneously changing the natural frequencies of the system. We call it a nonlinear harmonic system. Okay. Sorry guys, I digress.

Wow!
Don't forget to take the bifurcated dodecahedral 2nd derivative of the cubed root of the dihedral bilateral divided by the moment of inertia times the section modulus.
I think that may reveal the answer.

[luket]

Sorry to offend you Dave. I'll try to use smaller words in the future. Just thought it was important that the vibration disappears suddenly as rpm goes up.

To summarize, it seems like something supporting something rotating has slop.

[81SPIDERMATT]

you stated that you feel it in the shifter ... is that because you have not had an extra set of hands or feet to be elsewhere and try to feel the vibration ... or is it only in the shifter that you can feel it .... either answer would not lend my inexperience to a conclusion but it may help those smarter than me try to narrow down the possibilities

edit ...could a loose exhaust mount somewhere cause vibration ?

[majicwrench]

Sounds to me like you have something out of balance with the engine.

Mounts exhaust etc will transfer the noise/vibration to the body. If shifter vibrating, body not, you have balance issue.

[focodave]

Sorry to offend you Dave. I'll try to use smaller words in the future. Just thought it was important that the vibration disappears suddenly as rpm goes up.

To summarize, it seems like something supporting something rotating has slop.

luket,
I hope you know I was just playing. I honestly meant no offense and certainly didn't take any offense.
I love the technical talk (I am an engineer by degree).
Seriously, though, I hope you find out what it is.
I know how annoying those things can be.
I'll definitely be following this one out of curiosity.....