Has anyone had this problem before? I was driving my 73 dart on the road today and all of the sudden I start hearing a loud gear/humming noise resinating from under the car. It almost sounded like tire tread on a big truck...you know that weird humming noise you get from big off road tires? Well, I put the trans in neutral and the noise went away. Then I put it back in gear and it came back. The car doesn't perform any less, but I don't know if the trans or the rear end is the issue ? I checked the fluid in the trans and it doesn't smell and it looks super clean... I neep some suggestions! Help...

In neutral you have no load on the drivetrain. Does it vibrate also? It could be as simple as a u-joint. Could also be a rear axle or pinion bearing. Bearings don't usually go all of the sudden,they progress and get louder. need a little more info. Is it worse under load or coast?
Doug

There's no vibration, just the humming noise while driving. When it's just coasting in neutral the noise goes away. It seems to happen under a load, but like I said it just happened all of the sudden and it's driving me crazy.

Sounds like typical rearend whine, but interesting that it seems to have shown up all of a sudden. Has the rearend gear been in it for a while?

Yes, the rear end has been in there for a while. About 5 years ago I had a 9" thrown into the car with 3.55 gears. I had the entire rearend rebuilt and I can't imagine it being bad already especially that I only have about 6k miles on it. I'm going to check the fluid in this week...

The other thing I should have mentioned is that I had the 904 trans rebuilt about the same time I had the rear end swapped out. The trans has a B&M shift kit in it. When I got the car it had a 318 w/2bbl and I upgraded to a 360 w/4bbl with all the bells and whistles. I've been having some issues with the kick down linkage and shift points. One person thinks it might be the valve body in the trans but I think it's just the linkage. Is there a correlation with all of that?

Does the noise change when turning a shallow turn right or left? Is the noise a whine or a growl? I can't see "gear whine" starting if it was quiet before. Experience could tell if its drive shaft speed or wheel speed. I doubt if its in the trans. I would first pull the drive shaft to get a good feel of the u-joints. See if theres any bind when they're twisted back and forth. If there good pull the diff cover and look for metal debris.
Doug

I'll give it a shot and let you know what I come up with. I plan on putting the car up off the ground to test that theory... Thanks!

Well, I lifted the car off the ground, check the gear oil (it looked great, not dirty, not smelly) and it was full. I checked the drive shaft U joints and they are good. The noise sounded like it's coming from inside the read end. When I check the gear oil, it was clean and there were no metal shavings in the oil. I also checked for leaks and the axles are solid, no movement up or down. It's just a weird noise under a load while driving. It feels normal to drive, the noise just comes, and goes (learned that from the drive today!)
Question? If I drive it like this, could it eventually freeze or lock up while driving? What do I do? Drive it until something goes wrong or start throwing parts at it?
I'm up for more suggestions..

i would guess an axle bearing if its that u can usually feel the outside hub it will get hot if its that it can lock up but i have never worked with a 9"

What rearend is it?. It can come on all of a sudden like it wasn't there the last time out. If you can stand the noise expecially by knowing that it isn't going to blow/lock up it'll last a long time.