2001 Mazda 626

Just bought used Mazda 626. Heard loud rattling noise coming from belt area. Noise much louder when air conditioning engaged, but only occurs at low rpm. Can still hear noise at low rpm without air conditioner on. Coolant was low (thus engine would get hot on 87 degree day) but when I added coolant, the coolant level remained high after driving for several miles.

It is the next morning after buying the car and when I cold-started the car, the noise had disappeared. Thus, the noise appears to be related to when the engine warms up.

Help!