strange noise what could it be??

[84 400]

I know It could be alot of things but thought some of you 4x4 guys might be able to point me in the right direction....

Truck
1977 K10 Big 10 350 Th350 Np203 10 bolt font 12 bolt G80 rear. Still full time 4x4. 28,000 miles all fluids where changed by me in the last 2 months and no leakage. Also checked levels yesterday, greased all points and drive shaft slip yokes.

Strange piece of plastic
While greasing the zerk fittings on the front and rear drive shafts I was wiping off some of the grease and a large half circle shape plastic chunk was inside my paper towl. I could not figure where it had come from but did notice the only plastic injectod area in the drive line was the CV joint on the front drive shaft right before the transfer case.

The Problem
when driving down the road I get a large humming noise and possible a slight vibration. Feels like it is coming from the T case or Front end but it is hard to tell. Kinda sounds like a louder version of tire hum but the pitch changes with speed. it starts around 30 MPH.

A few Questions...

1) How do you inspect / check U joints. I have replaced them before but Don't know How to spot a bad one.

2) I have heard NAPA sells a rebuild kit for the CV joint on the font shaft. Can it be rebuilt or should I be looking for a new shaft if it is infact the problem?

3) On an NP 203 where do you drain the case from? I have it full of 30 w motor oil as recommended in the manual but would like to change the fluid. there is no plug so do I remove the rear inspection cover??


Thanks for any info