Transmission humming noise between 40-50 mph

[Modoyadney]

Hi all,
Had my first longer test drive with my 99 Suburban today and unfurtnately I noticed a humming noise which must come from the transmission.

It occurs between 40-50mph and is the loudest when cruising. As I accelerate it gets quiter or dissapears if I put the foot down a little more.
If I don't step on the gas pedal, it also gets quieter or disappears....

Overdrive seems to have nothing to do with it, because there no difference between driving in D-Overdrive or put in regular Drive (3).
ATF level is full.

 

[Modoyadney]

I also noticed that the ATF color is clear bright yellowish. I've heard from some people that there are oil manufacturers that don't stick to the color coding, but I'm gonna change the transmission fluid next week anyway.
Just hope that if it was a wrong ATF, it didn't do too much damage...

 

[Blackcar]

Transmission fluid should be red in color. Yellowish sounds like engine oil.

 

[mikeyss]

Could it be the rear pinion bearing going bad in the rear axle?

 

[Modoyadney]

Could it be the rear pinion bearing going bad in the rear axle?

After I drove the truck again, to figure out where exactly the sound comes from, it sounds like it would be the middle of the car.
It doesn't sound like it would be in the rear, but I'm not 100% sure.
It starts around 45mph and is the loudest at 50mph. it's completely gone at 60mph and more...
When I step off the gas pedal, it definitely disappears.

I changed the rear axle fluid last week and didn't see anything suspicious in there.
Transmission fluid will be changed next week.

The pinion bearing is a ton of work to replace, isn't it?

 

[exp500]

If you are 4x4, change Tcase fluid and inspect old for excess Glitter.

 

[tom3]

Yes the pinion bearing is, but if the lube/fluid was nice and clear I'd say the noise is not back there. Never seen yellow transmission fluid in a GM product.

 

[Modoyadney]

It's a RWD.
The color worries me a little, but I will replace it as soon as the new filter arrives with regular Dexron III.
At least it doesn't smell weird and seems to have the viscosity of a regular ATF...

Is there any way to properly check the pinion bearing. Wouldn't it become louder and louder as speed increases?

Thank you all for the help.

 

[mikeyss]

I'm not sure how to check for the pinion bearing. I remember when I had my 05 Tahoe, at around 40 mph it sounded like someone blowing air across the top of a glass coke bottle. I replaced the rear axle, and the noise was gone.

 

[Eman85]

You can grab the pinion yoke and try to move it to feel for play but if it had play you'd have really heard it. Pinion bearing will usually make noise on either accell or decell but it will be consistent and speed doesn't matter as it's the force applied to the bearing from the pitch of the gears that makes it noisy. Yellow fluid in the trans is different. Some use hydraulic fluid instead of trans fluid, why? I have no idea but they do. Different theories like a lot of things you'll see in auto repair. Transmissions usually don't make a humming noise, I'd say rear if I was guessing.
Pinion bearing is a little bit of a pain as you have to install and crush a new crush sleeve.