Strange noise

jscott · Nov 17, 2024

On my way home yesterday, after driving the car for 2+ hours I started to hear a noise. It sounded like if you were driving past a saw mill, a high speed metal noise that would come and go. When I would take my foot off the gas it would stop immediately, Revving the engine in neutral (fmx auto) revealed it was not coming from the engine. To me (poor hearing) it sounded like it was coming from the rear, but it was not constant, would start then after a few seconds fade (thus the driving past a saw mill thought) It would happen mostly at 44mph-60mph but also heard it lower once or twice. I have been waiting to get her into the shop for a diffy seal replacement (slow drip leak) as well as an output shaft seal replacement (slow tranny fluid drip). If not the engine what could it be that would stop when not in gear?? Thanks for your thoughts.

Sheriff41 · Nov 17, 2024

Check your u-joints.

Tnfastbk · Nov 17, 2024

sounds to me like a bearing trying to seize up somewhere. I know an alternator will do that only when it is charging at time. But could be many things. Let the hunt begin. Keep us posted. Just check the simple things first.

jscott · Nov 17, 2024

My first thought was a bearing but why would it stop as soon as I take my foot off of the gas?

Tnfastbk · Nov 17, 2024

like previously mentioned, check your u joints and could be something in the rear axle/pinion which would unload whenever you let off the gas. Not in gear would also remove the load from the rear axle, be it the pinion, u joints or whatever the case may be.

Sheriff41 · Nov 17, 2024

jscott said:
My first thought was a bearing but why would it stop as soon as I take my foot off of the gas?

When you take your foot off the gas, you are changing the load on the drivetrain.

Don C · Nov 17, 2024

One of the main causes of a differential seal leak is a problem in the differential, usually the pinion bearing going out or loose.

Hemikiller · Nov 18, 2024

Grease your u-joints, If it still makes the same noise, then you'll need to look at the differential. You might have a pinion bearing on the way out. Those bearings see a tremendous amount of load, which changes on throttle position.

rcadd1ct · Nov 18, 2024

Check fluid level in the rear end, too

69 Rustang · Nov 18, 2024

Sheriff41 said:
When you take your foot off the gas, you are changing the load on the drivetrain.

Agreed, it could be something in the engine, but without load it may not manifest the symptoms.

basstrix · Nov 18, 2024

As has been implied or stated, it sounds like a pinion bearing. Get that checked ASAP as it will destroy the ring & pinion if it hasn't damaged already (assuming that's what it is).

jscott · Nov 22, 2024

I was finally able to get under the car today, I wanted to check the fluid level in the differential. I guess the only way to do that is the racket size plug in the driver side, correct? I pulled it and fluid started to run out so I put the plug back in. Would that say I have enough fluid in the diffy then?

Sheriff41 · Nov 22, 2024

Sounds like you diff is filled.

rcadd1ct · Nov 22, 2024

jscott said:
I was finally able to get under the car today, I wanted to check the fluid level in the differential. I guess the only way to do that is the racket size plug in the driver side, correct? I pulled it and fluid started to run out so I put the plug back in. Would that say I have enough fluid in the diffy then?

Check it with the car in level ground. Axles not drooping.

I drive up on ramps in the front then jack the back up and set the tires on a second set of ramps

Yes the fill plug is also the check hole.

jscott · Nov 22, 2024

rcadd1ct said:
Check it with the car in level ground. Axles not drooping.

well I don't have ramps. If I put the front on jackstands then use a floor jacks to jack up each rear tire to normal hight would that work the same? After the axle is on jack stands of course.

rcadd1ct · Nov 22, 2024

jscott said:
well I don't have ramps. If I put the front on jackstands then use a floor jacks to jack up each rear tire to normal hight would that work the same? After the axle is on jack stands of course.

Yes. That would work.

Just need to get it up in the air to get under it and have the weight on the rear axle and the car level.

Some people make wooden cribbing and put all four tires on them.

Whatever works.

tpj71mach · Nov 23, 2024

I was planning on starting a thread to ask about this exact same issue. It is like jscott is reading my mind, haha. I have an FMX and the differential had a slow leak at one time and I have had a similar noise starting on my car just recently. The rear differential is one thing I have not addressed yet in the restoration. When I bought the car, the pinion was leaking out of the center of the yoke and a friend of mine came over and sealed that for me temporarily and we filled the differential hoping it would be good for a little while. I had no idea considering that I bought the car with no engine if the differential was any good at all. I would assume that needs to be the next thing budgeted for.

tpj71mach · Nov 23, 2024

My car came with a 3.0 open case rear differential, and my plan was to try and get the resto completed and drive it a while and eventually upgrade to a complete 3.50 limited slip unit after I saved up some more money, haha.
When I first drove the car, the differential had a little pinion noise on load but not awful. That noise has not changed in the over 500 miles I have driven it since getting it together in the fall this year. Recently I have been hearing a noise in the rear only on acceleration and gone when you let off the gas. At first it almost sounded like a belt chirp except from the rear. I thought it might have been a vapor leak because it almost sounded like a whistle. My gas cap seal was deteriorated so I replaced that thinking it could be causing the noise but no change in the sound.
I planned to start a string asking what you guys thought the noise could be and then I saw this thread from jscott. It sounded to me like they are describing the same noise I have been hearing. I know my differential is full and I installed new u-joints and greased them when I assembled the car, so I guess I have to assume that it is the pinion, and it will need to be replaced, right?

basstrix · Nov 23, 2024

jscott said:
well I don't have ramps. If I put the front on jackstands then use a floor jacks to jack up each rear tire to normal hight would that work the same? After the axle is on jack stands of course.

For troubleshooting purposes, you've confirmed the rear diff has oil. Next, I suggest you check the pinion for end play. WIth the transmission in neutral and emergency brake set, grab the pinion yoke and rotate it back and forth. Pay attention to the forward/backward motion of the yoke. There should be none at all. In the early stage of failure, there may not be much play.

As for failing u-joints, my experience is they often produce a chirping sound when you first start to accelerate from a stop. You may also hear a clunk when going from a forward gear to reverse and vise versa. While you're under the car, look at the u-joints for powdered rust residue where the cap and trunnion meet. Also check them for play.

My money is still on pinion bearing, based on your description.

Edit: A loose pinion nut will also produce a whine that is load dependent.

cjstaci · Nov 24, 2024

Sounds like a pinion bearing and a leaking seal could be a good indication. When checking your gear oil, look for metallic particles.

Strange noise

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