drivebelt issue
 

hi,
I got a 1995 mustang V6, I started hearing a kinda vibrating noise, more when I accelerated. I went to the shop and they told me I needed a new drivebelt, so I changed it and it worked fine for 2 days no noise. Then today I heard a noise again but this one is more constant and annoying. Could it be the drivebelt again and that the mechanic that worked on my car messed up (or purposely done this to get me to go back again) or could it be another problem. When i poped the hood it sounded like it could be the belt.
It just doesnt make sence that it would be the belt again after 2 days. Thanks for any help you can give.