Finally got to go for a ride and 2nd and 3rd gear on the trans is very noisy.
1st and 4th sound fine. There is no slipping and it seems to run fine, but its loud. I changed the oil and no difference.
70 Challenger 4 spd.

Am I looking at a rebuild or something simple?
Thanks

23 spline or 18 spline?
18 splines are more noisy than the 23's.

Depends on what body platform and year , and if the trans has never been messed with.

What is the history of the trans , has it ever been rebuilt ? 2nd and 3rd will have will be side loading the bearings more so I would suspect a rebuild is in order , the trans that was in my 69 bee sounded like it was full of rocks . did a rebuild and it quieted it down so much I could hear the wires under the dash rattling ...

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Is there a way to tell without dropping the tranny?

1st and 4th are quiet, just 2nd and 3rd are loud.

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Without pulling the trans, no. What rearend, Dana or 8 3/4? Most 18 splines had Dana's behind them.
18 splines will "whine in 2nd and 3rd due to gear angle (cut). This is nothing to worry about.

The rearend is 8 3/4. 489 with 355 gears

Need a little history on the trans. With that rear it should be a 23 spline and not noisy in 2nd and 3rd. Has the trans been replaced at any time? Who installed it? What clutch was used? Was the trans sitting for a long time before install? If all the answers point to the trans being a 23 spline then I would lean toward either the front or read trans bearing being bad. The trans will have to torn apart and at that point I'd go ahead and change them both.
What gear oil did you put in it?

Bouht the car in pieces, so no history on the trans. I was told it was in good shape. The previous owner installed the trans and clutch (hydrolic)
It might of been sitting, dnt know, but it is date correct for a 70.

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Bouht the car in pieces, so no history on the trans. I was told it was in good shape. The previous owner installed the trans and clutch (hydrolic)
It might of been sitting, dnt know, but it is date correct for a 70.

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In that case, the only real way of finding out what is causing your problem, is to pull the trans. It could be some "junk" had built up on the gears over the years. If that's the case, just driving it will eventually clean it off and the gears will quite themselves down. Could be also, that the bearings are "pitted". If this is the case then they will have to be replaced. It's hard to diagnose problems without the right info.
My . Drive it a bit. If the noise gets worse, pull it. If it gets better, drive it.
What gear oil did you put in it?

80w 90 gear oil
I will drive it and see if that helps.
Thanks for the advice

The tranny sounds much more quiet that I've been driving it.

Hope that is a good sign.

My money is on the side shaft rattling around.

Just going to drive it, look at it in the winter.

What do you mean side shft rattling around?

My old aluminum cased 833 OD had a side shaft that beat up the case. When in 2nd or 4th (old 3rd gear position in a non-OD) it would rattle and howl loudly. Popping it into 3rd (4th on non-OD) it was smooth and quiet.

If the trans. continues to get better drive it,but change the fluid soon If you want to see what is causing the noise now pull the tranny out and look inside it for rust and pitting on the gears, especially 2nd and 3rd If there clean and not pitted then it is time to remove the output shaft,tailshaft so you can pull the cluster gear out and look at the cluster gear shaft for wear If you have never done a stick tranny like the NP833 then it is time to learn or take it to a good shop and ask to watch so you can learn how to do it yourself Thier not really hard nor are they way easy But there not as hard as a Mopar 727 or a 904 automatic. Let alone a Ford C6 or a GM Turbo 400 If you take it apart there are two roller ball bearings, one in the front on the input shaft and the other on the output shaft. Both need to be check for roughness when spinning, you can also look inside them for pitting(after cleaning them up) on the inner and outer races and the balls themselves If there is any pitting or roughness replace them Good luck