I'm hoping someone has either had this issue before or can help identify based on sound.

I've taken my 1995 base to multiple mechanics and nobody can figure it out so far...but when I turn left or right very sharply, it moans (see video). I just started doing this not too long ago and I'm not sure why.

Supposedly, no rubbing (which is what I thought, but last mechanic had it on a lift and showed no rubbing), so I don't think alignment is the issue. I thought the sound was coming from the front, but when sitting in the passenger seat, it actually sounds like the rear, which makes it even weirder to me?

I did change the rear differential fluid not too long ago...would that cause this and only when turning? Seems like it wouldn't to me. I used Valvoline SynPower 75W-90 Full Synthetic Gear Oil and it says it has limited slip additive.

Here's a video of the sounds...the sharper the turn, the louder the moan. Small turns make no noise at all, that I can tell.

 

I would bet on power steering pump. How is the oil level?

 

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The power steering fluid looks good and at the right level, but yeah guess it could be noise bouncing since it sounds like it's coming from the back when in passenger seat. (mechanic is the one that showed me that)

Yeah, my car is a stick shift so going to have to get the wife to sit in there with me to show better. I was trying in the beginning to hold it towards the back to record how it sounded from there. Even in the video (with me sitting in driver seat), still seems louder in the front IMO.

You happen to know if you can really put too much of the additive? Or can I just crank it open and add a tube of additive to what's already in there? I went with this Valvoline based on it having it already in there, so didn't buy a separate pack of it. If that'll be too much, then yeah I'm going to just drain it all and try that if nobody else has any ideas.

 

if its a manual transmission it sounds like gear noise in the trans. Check the fluid level and for sparkley things.

 

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It is a manual and last time I changed it, I used the recommended (by most) Amsoil. It's been less than a year since changed, but I can check that.

Alright, back up on the jacks it goes! I'll check the fluid levels with those and add a pack of additive to the rear diff to see if it goes away...if it doesn't, guess I'll be changing some fluids again. Nothing like an older car to keep things exciting, I guess!

 

Alright, finally an update!

It does seem like it's the rear differential to me. I ended up adding some more LSD additive (K&W Posi-trac Gear Oil Additive) and the sound went away. Unfortunately, I didn't drive it a ton, so not sure if it would stayed quiet or came back (more on that below).

Since I'd read that you ideally don't want to mix additive with a product that already has some, I decided to go ahead and empty the diff out again and this time went with Mobil 1 75W-90 Synthentic Gear Oil. (Previously used Valvoline)

It says GL-5 rated, which to my understanding also means backwards compatible with GL-4. Bottle says it has limited slip additive added.

I drove around for a while and no sound! But last night, after about an hour of highway driving, I heard the moaning again...so thinking there's something not right about the fluid again.

There was NO sound before I changed the fluid, so I'm kind of regretting it right now...only did it in order to make sure everything in the car was good, since I didn't know when most fluids had last been changed.

I recall the fluid being red in color when I took it out, so I'm pretty sure it wasn't royal purple, at least.

Anyone ever changed the rear diff fluid in a MANUAL 1995 C4 or have any recommendations on what I should try? So much conflicting info online. I see where some say it should actually be 80W-90, but that doesn't seem available in synthetic.

I mention manual because I read online that manual C4's should only use GL-4 rated, but again...not sure if that's accurrate.

The car only has 9300 miles on it right now, so I doubt the rear diff is hosed from usage or similar. And, like I said, didn't do this until I changed the fluid.

Edit - I can also make another video if anyone thinks it'll help, but mostly just looking for recommendations on what fluid to use...gear oil smells like death and I'm tired of changing it out. lol

 

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Yeah, I normally try to do that, as well...but can't find this stuff anywhere easily.

Looks like only Summit Racing carries that? And when I go to parts.chevrolet.com to get it from my local dealer, it technically says, "Does Not Fit 1995 Corvette."

Do you happen to know or remember (if you've changed) if the color of that one is reddish? Like I said, shot in the dark, but trying to at least find one same color, hoping it's same as what was in there before. The Valvoline and Mobile 1 were mostly clear.

Thanks for the suggestion, if nobody else has an idea on what else to try, I'll order those from Summit or my Chevy stealership.

Edit - and if it helps for any other suggestions, the Mobil 1 does work better than the Valvoline did. The Mobil 1 is fine after starting and driving for a bit...doesn't seem to start moaning until after highway driving (or maybe longer city, haven't tried). The Valvoline started moaning right away.

 

I found both of those products on Amazon from numerous sellers at a wide range of price.
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Sounds good, thanks for your help!

 

Closing this out, in case anyone else has this issue, this is what ended up working for me: Red Line Synthetic Gear Oil GL-5+ 75W-90 1 Quart 57904 | O'Reilly Auto Parts

I spoke with the owner before the previous owner and that's what he used and it was good to go!

I'm willing to bet Royal Purple or Amsoil would've worked, based on how many people seem to have used them...it's just my local auto parts stores didn't have the RP in stock and I didn't want to wait for the Amsoil (probably a mistake). Maybe I'll give those a shot in 30-50k miles, if I'm feeling like changing this out potentially multiple times again. LOL

Also willing to bet the GM/Delco replacement would work, too.

For anyone tackling this job, I'd highly recommend NOT going with the Valvoline, even though it says it's a 75W-90 gear oil with limited slip additive...it was definitely the worst in my car.

The Mobile 1 also says all that and was better than the Valvoline, but still blatantly didn't work well.