Screech at 4200-4500 rpm

[RRoller123]

I have a screech that the car passes through at about 4200-4500 rpms. Others have mentioned a similar thing. Car runs fine, not engine related. Sounds like a cat whose tail is stepped on, or a metal on metal screech, chalkboard like. Awful. It is very brief and the engine passes right through it and goes on normally to higher revs with normal operation and power. It is NOT in the engine bay, and NOT a belt or any of the usual stuff.

I thought it had something to do with a harmonic vibration, but I can't pinpoint a cause. Exhaust has been thoroughly checked, nothing loose, nothing hanging making contact. E Brake line not making contact. It is very consistent, happens every time the engine passes through these revs, is always the same volume and intensity, and is always in the same narrow rev band.

I am now thinking that there could be a harmonic screech coming from the driveshaft support bearing at those revs.

So my question fer the here assembled masses is:

Is there any way to properly lubricate that driveshaft support donut bearing? I am tempted to soak it with some penetrating oil spray or silicone, while it is in place, maybe slowly spin it with the car up on jack stands while doing so, and see if it helps. But my understanding is that this is designed as a sealed, no-maintenance bearing? Mine has only a very few thousand miles on it, so it is not an age related failure issue. Pita to replace it, I would rather avoid that if possible and lubricate it if that can be done.

Thanks.

Pete

[Dawgme85]

What you describe doesn't appear to be driveshaft-related, as it would then occur at a particular SPEED, not RPM. It sounds more like a bearing that is (very) worn or lacking lubrication, to me. If it's coming from below, perhaps the throwout bearing is due for a replacement? In any case, I would be looking forward of the transmission input shaft, given your description. Good luck.

[RRoller123]

Thanks for responding. The Throw out bearing was replaced last year along with the clutch and addition of a lightened FW, plus seals, and all have been quiet and trouble free. The screech does not occur at all when I rev the engine at rest in the garage, so maybe you are right and it is in the transmission somewhere. I.E. It only occurs under load. Thinking about this, I only pass through that rev in 3rd gear going up, never been that tall in 4th, and there is no noise when I use the transmission to brake coming down, although it is rare that I let it go that high on a transmission brake. But it has never screeched coming down is the point, anyway.

But I am pretty sure that it does not originate in the engine itself. Seems definitely to be coming from somewhere under the car, roughly midsection. The rear engine seals leak a little, and the engine hasn't yet been pulled (by me anyway), as I only did the top end last Winter, with it in place. So maybe you have hit upon it.

I may spray the load bearing anyway, what the hell, it couldn't hurt it could it? Will be time to pull the transmission next Winter.

Pete

[azruss]

first, determine if it is engine or drive line. Does the sound reproduce in 1st and 2nd gear as well as 3rd. I wouldn't spray any lube at the driveshaft carrier bearing. That is a sealed bearing and all the lube will do is cover the rubber in the pillow block. Could make the rubber swell and get soft.

[RRoller123]

Good idea. I went out and ran it up in first and second gear and got the noise too, so it isn't gear dependent. I didn't run it up that far in 4th, summer traffic here doesn't warrant those speeds. I am not sure I am properly describing the sound, either. It is sort of like the noise a rattle snake tail makes, but mixed with a screech. And of very small duration, just a 1/4 second or less as you pass through the rpm range. Immediately starts and stops predictably. I would think a dry bearing would be noisy more of the time and across a wider rpm range. Seems harmonic related to me.

All exhaust, manifold, cam bolts, everything I can see or reach is properly tightened.

I am going to pull off the heat shield that is over the exhaust manifold and see if that helps isolate the source. Maybe the two layers are unhappy with each other. I need to take one of the kids out and have them lean out over the side and listen and see if they can isolate the source location a little better. My ears are so bad I could place the sound in another car nearby. Oddly enough.... I had an odd chirp/screech coming from the AFM door a couple years back, when it was just barely touching the housing floor at certain speeds. I need to take a look at this again. Not likely, but one never knows. There is precedent there. (a side FYI, I ran the car at that time with the AFM cover off, just to watch the action of the AFM wiper arm. I was surprised to learn that the arm swings around quite a bit even at steady rpm. I expected its reaction to be pretty much linear with engine speed. Not so.)

Thanks for the ideas! It will make itself known eventually, and I will report so it doesn't get left hanging. That drives us all crazy.

Pete

[RRoller123]

SOLVED.

I replaced the Catalytic Converter with a temporary test pipe and the screech is entirely gone. I do not know if it was internal to the CatCon or was coming from the mounting springs, but as they were very tight, I doubt they were the problem. There is no apparent rattle inside when I shake the CatCon.

The car overall seems to run somewhat noticeably smoother, at all rpms, and there is perceptibly more power available, noticeable on hills, where far less pedal depression is needed to keep up speed. Torque doesn't seem changed very much, i.e. acceleration is about the same, but there is better power at higher rpms (3500+) and better power on the hills. Overall smoother performance, which makes sense as the system should flow much smoother with the elimination if the turbulence that was being introduced by the CatCon.

No change in audio noise at all, if anything, it sounds a little better, but certainly it is NOT louder. Still produces that wonderful Italian hum.

Pete

[Nanonevol]

So that'll be a "permanent" test pipe?

[RRoller123]

No, just temporary. I comply with all local, state and federal regulations regarding all of my vehicles.

[FiatMac]

SOLVED.

I replaced the Catalytic Converter with a temporary test pipe and the screech is entirely gone. I do not know if it was internal to the CatCon or was coming from the mounting springs, but as they were very tight, I doubt they were the problem.

I suspect it is the mounting springs for cat hitting a harmonic frequency. I had this on my 82'. Wrapped them with duct tape to act as a damper and the noise was gone!

[Nanonevol]

No, just temporary. I comply with all local, state and federal regulations regarding all of my vehicles.

Oh, sorry, of course. I wasn't aware of that regulation. It doesn't apply to my '77.

[RRoller123]

Hi FiatMac: "I suspect it is the mounting springs for cat hitting a harmonic frequency. I had this on my 82'. Wrapped them with duct tape to act as a damper and the noise was gone!"

I thought this too, and I think you may be right. BUT I reused the springs from the CatCon on the test pipe, and inspection of them showed no spots of wear anywhere on them, so I am still unsure of the ultimate source. The sound was significant volume, the best description I can give is a rattlesnake rattle combined with a screech, and very definitely harmonic related. Short duration as I oassed through the rpm. Does this match the sound you had on your '82?

Looking inside the CatCan, it was old and very carbon choked, so I still suspect something internal to it. I may put some compressed air through it and see how blocked up it is. I suspect an internal baffle had worked loose or something like that. At any rate, the car runs better, sounds better, and it cut a few pounds! The CatCon weight when removed was 10lbs-10ozs, and the test pipe is only 2lbs-14ozs. Almost 8 more pounds cut out of the car!

Pete

[FiatMac]

Pete, if I recall correctly this happened on mine when I replaced the muffler on the car. I had some trouble getting everything to fit and likely changed the tension on the springs which would have changed the natural frequency.

[RRoller123]

Rattled like rocks accompanied by a screech? Very specific, very odd sound. (would be good in a horror film, like when the shower curtain is pulled aside)

[barticus]

Glad I read your post guys! I have the same problem on a "83" and was getting ready to change out the worn Driveshaft support bearing. The noise drives me crazy. I'll check the springs and change the support bearing. If that doesn't work I'll replace the catcon. Thanks,
Bart