Did you supply the mechanic the parts or did they suggest the parts and source the parts? Warranty might not stand if you supplied the parts...
I installed the kit myself did the tension up and it was sweet, no noise or anything for a good year, recently i had to take the belt of and when i started the car it made that exact noise but much louder, i loosened the tensioner off and the noise has gone, im not worried that the belt will slip or jump and it was a Cu*t to even get off and back on again.
Next time i will just buy genuine Nissan belt as their is nothing is wrong with them..