If it is a sound that seems to be at a constant interval, it could be the bottom pulley (that is bolted to the crankshaft) that has come loose, and is hitting the fan.

Take a look at that from under the radiator area. If that is the case, there r 6 allen bolts that hold the pulley in place. These bolts have been known to break or come loose.

The other problem could be that ur vacuum pump may be coming apart. If that is the case, u have to address this problem immediately. Parts coming loose in vacuum pump can cause alot of damage.

Hope this helps.