Prob with Altima 2.5 after drive belt change

[sanjiv_sv]

After i got the drive belt changed at the dealer workshop, i experienced this problem. On cold start without the aircon on, moment i put the gearshift into drive, i hear and feel a strange vibration accompanied by a sort of continous creaking noise, but this is not so pronounced once the engine warms up.

With the aircon on, on the other hand, the moment i go into drive mode, the vibration and sound starts, whether the engine is cold or not. At traffic light stops, car park manouvering, it has become a nighmare and more than the vibration it's that creaking sound that makes me switch off the aircon even though the outside temperature is close to 47°C

I took the car back to the dealer, he said maybe the alternator bearing is gone and needs replacing, but said he would change the idler pulley and see if it helps. Now i have a new idler pulley and still the same problem. I dont believe that the alternator bearing is gone bad and hence the whole alternator needs replacement as the dealer gives me an estimated cost of $800-1000 plus labour.

can any1 help me diagnose the problem?? i suspect that when he changed the drive belt, he has screwed up something