Oh CRAP. That's something you don't want to hear.
If it whirrs or sounds kinda like a gear drive on a V8 or a supercharger, it means the t-belt is too tight. I did that one time. Had to tear everything off and re-tension it. I took it in to a Nissan dealership before taking everything back off and asked a head tech to listen to it. He concurred, it was too tight and the cam sprockets *can* break off the end of the cams if it's too tight.
Easy way to check tension on the belt: with the timing cover off, grab the belt between the two cam sprockets. You should be able to twist the belt 90 degrees by hand. No more, no less. The Nissan head tech gave me that tip.