Truck Repair: Re N14 Accessory Drive Shaft Braking

Question
Hi, I am having problems with the accessory drive shaft braking on my N14 It broke 4 Times last 3 times it was braking on the compressor side and last night it broke on the pulley side, I replaced it myself 2 times and had my mechanic replace it last time, We checked the bush bearing in the front cover seamed OK and also compressor turns OK we changed the dampened on the crankshaft that seamed to help a little bit I got about 3oookm more out this last one (3 or 4 trips and it breaks approx 4oookm) but now its braking behind the pulleys, WE have also replaced o rings on the compressor.

Each time we replaced the whole Accessory Drive assembly.
Also I have noticed that sometimes when the engine is idling the compressor instead of having even taking noise it sort of goes like tttt____tttt____  

If you could let me know thanks

Answer
The cheapest way to start is pull your timing cover and pull your pan and valve covers. I would start rolling the motor by hand feeling for anything out of the ordinary and checking everything over. Has to be something bad somewhere else to cause it to vibrate and break. Are you replacing with a factory part or aftermarket if aftermarket would try using factory part. As for the compressor sounds like maybe a valve is letting air past it wouldn't hurt to change your compressor to cure the sound and to also see if something internal on the compressor isn't causing you shaft breakage either. More I think about it if I come up with something else i'll let you know but for now I think you just need to start pulling covers and checking everything out.