If you replaced it with the correct belt, then the Idler Pulley needs replacing. Spring is broken or weak.
Had the same problem on a 2001 b3000, it would clear up for a day or two after installing new belt then it was back. Changed pulleys and tensioner thinking that was surly it but again it came back within a week, slowly. finally someone told me try quality and it was all left to try so instead of buying my regular $17.99 belt which would have been my 4th in 6mths. i spent the xtra and got the $33.00 one and rented the tool to change it with correctly to ensure i did not stretch it any while putting it on and that alone made a heck of a difference but its going on a month now and not a peep out of this newer belt, im sure quality is the way to go, hope this helps.
I'm not much of a mechanic but, it may be too loose.
it could be your idle control
Lay still.
This may, or may not be applicable, depending on if you had tried replacing the serpentine belt also. One cause for this would be the belt-tensioner has weakened and is not applying sufficient tension on the serpentine belt, allowing it lose traction on the pullies.
It may be the serpentine belt. Sometimes when you purchase an "on the cheap side" serpentine belt, it has a tendency of making noise even when the tension pulley has been change. Try purchasing a more expensive belt. The diamond back serpentine belt may be the best solution for this problem.
Could be a fuse, could be broken wiring, could be a dead battery.
Have you replaced the cam and/or crank sensor?
I replaced ac clutch and power steering pump and serpentine belt due to squeal upon starting engine. Also when turning wheel to full range. Took belt off and cleaned pulleys with a small wire brush and brake cleaner, been 10,000 miles and quite as the day I first bought it.
Inside the alternator
wheel bearing
It could be anything from the engine to the connectors
I recently had a water pump replaced in my 1999.5 Jetta. The pump is located in the block with the timing belt. It is recommended that the water pump be replaced at the same time as that belt because of where it is. Vw did this so that if the serpentine belt broke, you could still get to safety without warping the engine block because of over heating because the water pump was no longer functioning.