You may have a worn serpentine drive belt that is slipping or worn out. First, check its tension to see if it up to speciications, and if not, tighten to spec. If that that does not take of the problem, inspect the belt.

The flat serpentine belt has several small "v"s on the drive side. These function the same way that the older fashion single v-belt did, only these are smaller and more of them. Only the Vs should contact the sheaves; the backing should ride above the sheave ridges.

When the Vs wear enough to let the backing ride on the sheave (pulley), the V sides looses their grip and the belt slips. Its worn out at that point - replace the belt.

To get a view of the belt fit on the sheave, use an inspection mirror and a light. Naturally, this inspection is accomplished with the engine stopped.

Sometimes, the sheave (pulley) can give a clue. If the belt has been riding and slippng on the ridges of the sheave, these edges may show evidence of such contact. Normally, these ridges do not contact the belt have oxidation tarnish.