vibrating noise from pulley

hi, i installed a high out-put alternator in my 96 sl2 and it came with a overdrive pulley. ( it is 2 inches in diameter). and in reverse or when the p/s turns on i would get a vibrating noise coming from what appears to be the alternator pully area and i was told from the manufacture that it should work with a stock belt but i am wondering what could cause the noise?

I don't see how using a smaller pulley would allow using the stock belt. Try finding a slightly shorter belt. About a 1/2" shorter. The parts counter guys should be able to get one for you.

ok cool, which brand of belts would you recommend, i like goodyear but the next smaller one is a .25" smaller, then it jumps to 2.5" so i dunno. do you think half a inch is good. because i noticed what with a brand new pulleys. ( idler and auto tensior) that it won't make the noise until a couple of days later. so the stock belt is not that loose . just enough to cause the noise. do you know what the stock size pully is? i tired measuring it while still in the car but i can't quite make it accurate and me plus working under the hood equals half noob.