Whining Sound at 50 mph - 1995 Chevy C1500

I’ve got a noise that has me puzzled on my 1995 Chevy C1500 (2wd). I hear a mid to high pitched noise when I’m driving at between 45 and 55 mph (70-90 kph). It varies in pitch depending on the speed I’m going, starts at around 45 and goes away above 55, and also tends to go away if I lift off the gas. The sound occurs in any gear, but always at the same speeds.

My leading suspect is the differential, but I haven’t ruled out anything yet. I pulled the differential plug and the fluid isn’t very dark.

Anyone have some experience with this?

I’ve noticed the same thing with my 94 Suburban but the ‘sound’ seems to be louder when going downhill on a low tank of gas…I’m wondering if it’s the fuel pump?

My limited experience w/differential whining (on my 70’s Ford truck) is that it just gets louder and more annoying the faster you go. So the fact that it quiets down above 55 seems inconsistent with a problem in the differential. Ask the shop to look under the car, do a visual on the drive shaft, see if thee is something loose or hitting, the flexible drive shaft joints are not getting loose, etc. Ask them to lube what needs lubed while they are down there, maybe that will fix things. Worth a try.

I tend to lube the chassis/driveline every oil change, so it shouldn’t be lacking in that regard. It’s a very constant tone (similar to a whistle) if the speed isn’t changing, so my thought is that it will be either one of the bearings on the output side of the transmission, or the ring and pinion. I’ll take another look for driveshaft interference and double check the U-joints.

Another note I forgot to mention, the noise occurs regardless of outside temperature, or vehicle run time. It’s been doing it for the entire time I’ve owned the truck(a little over 2 years), but the volume and tone of the sound hasn’t changed in that time. The fuel level doesn’t seem to effect the sound at all.

I guess if I can’t find anything, I’ll pick up an IR thermometer and take it for a long drive at 50. Where there’s noise, there’s friction and heat.

You may want to check the differential for wear, excess play in the pinion or abnormal patterning on the ring and/or pinion gear.

Do you know anything about the truck’s history?

What’s the truck’s payload? Heavy duty truck differentials will often have noisier rearends than light duty vehicles.

It’s possible the differential is just worn out and kaput, sure. Maybe you can’t hear the noise above 55 mph because other road noise and wind take over. In which case you need a new differential or have the one you got rebuilt. You could pull it out and see if there’s anything visibly wrong, like chips on the gear teeth. I did this on my Ford truck one time and it isn’t that difficult to get out. Getting it back in, what with the weight and it being coated with slippery oil … well, that’s another story … lol …

hmmm … still thinking … Since you noticed this problem when you first bought this 1995 truck 2 years ago, it’s possible there is something wrong w/the oil the prior owner put in the differential. Wrong type, wrong viscosity, heat damaged, water-logged, etc. As first order of business, it might make sense to drain out all the old fluid and put in new, the exact type recommended by the manufacturer. Sounds like you are a DIY’er, so this project wouldn’t be much of a time or $$ expense to you. Even it it didn’t work, at least you’d know the rear differential was properly lubricated going forward.

The IR thermometer is a good idea too. Worth a try. Another idea if you can source the needed equipment, I’ve heard of auto shops addressing customer noise complaints by putting wireless (I think) microphones here and there, all connected to an audio amp and headphones in the passenger compartment with a rotary switch, then they drive down the road and turn the switch from one microphone to the other to listen where exactly the noise is coming from. I think you’ll figure it out eventually, with some persistance. Best of luck.

As for the truck’s history, it’s lived its entire life up until I purchased it as a plumbing and heating truck, which means constantly being loaded down with tool boxes full of tools as well as various other somewhat heavy things, but no towing. Servicing would have been done by the local GM dealer.

It has a GM 10 bolt rear, 3.73 gears, non-locking.

I’ve been putting off opening up the diff because of the extended winter and driving it less than 100 miles/month, but I guess it’s time. Never hurts to know the fluid’s new. If I have to replace things, it’ll give me an opportunity to add a G80 locker so I don’t get stuck as easy next winter.

Thanks for all the help!

No problem, sounds like you have things under control, best of luck. Coincidentally, I was under my truck today checking the diff and xfer case fluid levels and lubing the driveline and steering gadgets. I have a locking 9" diff on my Ford truck, but if I were going to change it out, I’d put in a plain 'ol non-locker. The locker, aka limited slip does exacty that: slips. And when it slips, like when going from forward to reverse when parking, it jolts the driveline every time, causing me to have to replace the U-joints far more often than I would if I had a non-locker. Plus it makes a weird noise on slow turns as the limited slip clutches slip, esp when it is cold outside or the truck hasn’t been driven in a week. My Ford truck is 4WD so if the rear wheels get stuck I can always put it into 4WD, so I don’t really need a locker on the rear. I’m sort of a mininalist I guess. If I had my druthers about this truck, I’d change from a 302 V8 to the 2 hundred something V6, and instead of the auto xmission I’d prefer the 3 speed on the column manual, and the non-locking rear diff instead of the locker. One mod con I’d keep tho is the power steering. That’s a valuable convenience on a heavy truck. Oh, one more thing, if I had my druthers I’d prefer a manual choke too. I mean, how much effort is it to pull on that little knob, I mean really!