RoushNitemare

Hi, I just got an 07 Roush Nitemare F150, it has the Roushcharger on it and apparently the previous owner put on a smaller pulley. The truck came with a second belt so im pretty sure the belt thats on there is the right size for the pulley. Ever since Saturday night, theres a loud sqeak whenever its running. As the engine warms up it seems to get quieter. Anyone know what could be causing this?

Thanks,
Daniel