NP205 - Moving Noise

[NVBUYER]

Hey guys and gals,

I have a question that you might be able to help me with. When I'm moving and my speed exceeds ~15mph I get a weird hum/metallic sounds coming from the transmission/transfer case area. It sounds like a grinding noise (possibly a bearing?) from the outside, but a buzzing noise when I'm in the cab. Here's the kicker- when I put her in 4HI it goes away. Also when I have been driving it for an hour or so it will go away.

For further detail:
-72 k1500
-350 330hp/380ft lbs
-Level II 700R4 from Bowtie Overdrives with an AA adapter
-NP205 that I had 'inspected' by my local AAMCO before the install
-3:08/3:07 combo
-New driveshaft + joints

The transmission and engine have only 3000 miles on them. Has anyone heard this similar noise? Any ideas where to start poking around? Should I spare myself the trouble and overhaul the transfer case? I'm thinking it has 100,000 miles on it. Thank you for any input you can provide!

[bilfman]

A little hard to diagnose without hearing it. Try this to try and narrow it down. Go in 2 hi and lock the hubs does it still make the sound or go away.

I think that would for sure narrow it down to the tcase. In 4hi you are spinning the case with the tranny- noise stops. In 2hi hubs locked you are spinning the case with the front diff. I would assume that if the noise stops in 2h with the hubs locked the is is a tcase issue.

I have also had these types of noises from a driveshaft touching something. Exhaust etc. Look for any shiny lines on your shafts, that could indicate the rubbing against something.

[vectorit]

A worn shifter linkage will cause a buzzing noise in the cab.
When you are driving, put your hand on the shifter or your knee and hold it steady. Does the noise go away?

[snj8198]

Originally Posted by vectorit

A worn shifter linkage will cause a buzzing noise in the cab.
When you are driving, put your hand on the shifter or your knee and hold it steady. Does the noise go away?

that's what i'm thinking too. or if the linkage was assembled recently it could be coming lose again.

[NVBUYER]

Originally Posted by vectorit

A worn shifter linkage will cause a buzzing noise in the cab.
When you are driving, put your hand on the shifter or your knee and hold it steady. Does the noise go away?

I had thought this might be it and wasn't sure how to diagnose that this could be it. When I put my hand on the linkage it doesn't vibrate any differently than normal, any idea what else I could try?

[NVBUYER]

Originally Posted by bilfman

A little hard to diagnose without hearing it. Try this to try and narrow it down. Go in 2 hi and lock the hubs does it still make the sound or go away.

I think that would for sure narrow it down to the tcase. In 4hi you are spinning the case with the tranny- noise stops. In 2hi hubs locked you are spinning the case with the front diff. I would assume that if the noise stops in 2h with the hubs locked the is is a tcase issue.

I have also had these types of noises from a driveshaft touching something. Exhaust etc. Look for any shiny lines on your shafts, that could indicate the rubbing against something.

Understood. I'll do my best to describe it in detail. It sounds more of like a deep hum/buzzing in the cab. If I'm on the freeway and up against the jersey-wall I can hear the noise very clearly. It sounds like a grinding metallic noise (it stays for 3 seconds or so and then goes away- returning 20 seconds or so later) that changes in frequency with no rhyme or reason. If I leave it in 4HI without the front hubs locked it will not make the noise. In 2HI with the hubs locked it doesn't change the pitch and the noise stays. Of course in 4HI with the hubs locked it isn't there either.

I checked the driveshaft for rubbing and found nothing, it also seemed pretty rigid. Thank you for responding, I appreciate it!

[72BBCK20]

Originally Posted by NVBUYER

Understood. I'll do my best to describe it in detail. It sounds more of like a deep hum/buzzing in the cab. If I'm on the freeway and up against the jersey-wall I can hear the noise very clearly. It sounds like a grinding metallic noise (it stays for 3 seconds or so and then goes away- returning 20 seconds or so later) that changes in frequency with no rhyme or reason. If I leave it in 4HI without the front hubs locked it will not make the noise. In 2HI with the hubs locked it doesn't change the pitch and the noise stays. Of course in 4HI with the hubs locked it isn't there either.

I checked the driveshaft for rubbing and found nothing, it also seemed pretty rigid. Thank you for responding, I appreciate it!

based on the fact that there is no noise in the 4hi position seems like a shifter linkage issue or defective fork? Is the the grease zerk on the shifter pivot greased up, any binding?

[NVBUYER]

Originally Posted by 72BBCK20

based on the fact that there is no noise in the 4hi position seems like a shifter linkage issue or defective fork? Is the the grease zerk on the shifter pivot greased up, any binding?

No binding at all and it moves easily and freely. I think you're probably correct about the defective fork, but I won't know until I tear into it in the next week or so. The noise is quite unbearable at this point. I'll take some pictures (if I remember! ) and update this thread with the final diagnosis. Thanks for the input!

[snj8198]

so the noise has been getting louder since you started this thread? something must conking out quick.

[NVBUYER]

Well I told ya'll I'd report back on the cause of this - I eventually took apart and replaced all the ball bearings, (tapered) roller bearings, loose roller bearing and everything else in between including seals (in the 10 spline NP205 transfer case). Checked for wear and play, (forks and races were fine) to which I concluded the darn case was like new minus a little bit of gear play. I torqued all of the yokes, bolts and all the other goodies to about/near what they were supposed to be and put the transfer case back in. I repacked the double cardan and u-joints and checked my slip-joints. Drivelines checked out, u-joints were fine. Filled the case with some standard 80-90W gear oil and took it for a test drive. Guess what, the noise was still there.

Here's the fun part... When I was installing the t-case for the first (yes there were others but I won't even get into that haha) time I noticed a small rattle when I turned my front driveshaft; a quiet but noticeable metallic noise coming from the left front hub. I didn't think anything of it until I started discussing this whole issue with my mechanic last week. He kept it over night and called me the next day to report back what he had discovered. I should have investigated it when I heard it. The cam inside my Selectro Hubs had broke in half. [See attached picture] The noise was being generated by the sliding gear trying to engage while driving. If I would hit a bump, it would come close enough to put up a vibration through the frame and sounded like it was right underneath the cab. I hate to admit it but I was mistaken. So if you ever hear a noise that sounds like gear grind, check the easy/cheap places first! (Although I had both of the hubs apart 2 other times this winter, they were fine when I had them apart. I didn't think they could break so easily and it still has me stumped how it happened.)

So even though I have a nice and fresh t-case, I could have saved myself a whole heck of a lot of trouble. Thank you all for your help in the pursuit of this mysterious noise, and a special thanks to jbclassix for his detailed NP205 rebuild info found here [http://67-72chevytrucks.com/vboard/s...d.php?t=469726]. I really appreciate it. Hopefully someone down the line will be able to use this as a starting point.