My car makes a similar noise and it is the main belt tensioner. I agree with the previous posters--first thing to do is pull the SC belt and see if the noise goes away. If it does, the problem is in the SC or the SC belt tensioner. Mine did not go away, so I went with step 2 which is the mechanics stethoscope previously mentioned. You can stick it on all the pulleys (or injectors, etc.) until you find the one that is making the noise. Mine happens to be the main belt tensioner, and it sounds similar to your noise. I think these are about $100, but I have not gotten around to fixing mine yet. It is not going to hurt anything if that is what it is.
BTW: I got my stethoscope years ago at Harbor Freight. They are on sale right now for like $4, and they are always sending out coupons for 20% off of any purchase: Mechanic's Stethoscope.
