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Hi everyone, new Vette owner. I have a 1975 that runs good, but I'm hearing a little noise and have no idea what it is. When I'm stopped at a light, say, and I turn the wheel to move, I hear a slight rumbling sound beneath me. It only lasts for a second or two until I straighten out and drive, but I have no clue what it might be. I'm trying to learn. If anybody has any ideas on what it might be, it would be greatly appreciated. Thanks in advance.
Sounds like posi chatter. You drive the car, the diff gets hot, you stop at a light, take a turn and you hear and feel it. 75 diff's had issues with posi chatter as all Eaton posi's had, also with bad axles and ring gear bolts. They all can cause a noise when bad but with the other things the noise is constant, the posi chatter usually shows up once hot.
You can suck out the old gear oil add 2 bottles of GM limited slip and fill with Lucas 85-140 gear oil and see if that solves the problem. If someone worked on the diff or it has worn axles, it won't resolve the problem.
The noise happens when you're sitting still and turn the steering wheel? Both directions? And goes away when you straighten the wheels before rolling?
If so, it's got to be something with power steering pump or control valve -- as long as you do have power steering. First, check fluid level as Piersonpie suggested, then crawl underneath and check all steering linkage bolts/nuts for tightness. If you can have a partner turn the wheel while you're underneath to listen and watch, that will likely help isolate the sound.
The noise happens when you're sitting still and turn the steering wheel? Both directions? And goes away when you straighten the wheels before rolling?
If so, it's got to be something with power steering pump or control valve -- as long as you do have power steering. First, check fluid level as Piersonpie suggested, then crawl underneath and check all steering linkage bolts/nuts for tightness. If you can have a partner turn the wheel while you're underneath to listen and watch, that will likely help isolate the sound.
thank you, that's very helpful. I'm not schooled enough yet to get under the car and notice that.But I will tell the mechanic I go to .The power steering fluid is full.That's for sure.I will tell him what you told me.Thank you. The sound isn't exactly what I'm standing still.It's when I first start making the turn , and then when I straighten out , it's gone. It only lasts like 2 seconds, but it's a grumbling noise under me, not sure, if that makes a difference, but thank you so much.If that makes a difference, let me know.If you think it's something else. thank you
thank you, that's very helpful. I'm not schooled enough yet to get under the car and notice that.But I will tell the mechanic I go to .The power steering fluid is full.That's for sure.I will tell him what you told me.Thank you. The sound isn't exactly what I'm standing still.It's when I first start making the turn , and then when I straighten out , it's gone. It only lasts like 2 seconds, but it's a grumbling noise under me, not sure, if that makes a difference, but thank you so much.If that makes a difference, let me know.If you think it's something else. thank you
With your updated symptoms, this sounds like posi chatter, as @GTR1999 mentioned.
Sometimes a fluid change with the appropriate additive will make it go away.
If the noise is occurring on turns, then I'd listen to GTR1999 and Bikespace that the differential needs attention. New fluid and 2 bottles of GM posi additive is a good first step.
Last edited by barkingrats; May 14, 2026 at 09:50 AM.
Be advised:
There is NO drain plug for rear diff aka axle.
Gear oil must be sucked out of fill plug.
Then replace it FIRST w/ TWO 4oz bottles of the additive.
Then followed by filling with gear oil.
Same conditions on my 1975 coupe when I bought it. Changed the diff oil and added the aforementioned LS additive... noise gone. Then noticed the diff cover had a cracked spring perch, so all that had to come off to make things right.
Agree with everyone here.... change that diff fluid. Take a look at your power steering control valve and hydraulic ram.... but DON"T over grease or you'll blow the seals and have leaks everywhere.
It's an Australian trade secret to use banana peels to fix a differential. I'd still recommend an oil change and the correct GM additive, but perhaps @4-vettes can confirm that the alternative also works.
Grumbling noise underneath car
