1997 Ford Ranger XLT 2.3 5speed Making engine noise

Just bought a 1997 ford ranger with the 2.3 4 cylinder. noticed when at idle it makes a like ratcheting sound very faintly coming from the back of the engine bay like cylinder 1 when standing in front of the truck. When you rev the motor, it goes away, when you hold the motor at 1800 or more rpms its steady and perfectly normal but, on the deceleration, when it at around 1500 rpms it makes the ratcheting sound but more profound when held at 1500 rpms the sound will stay consistent and be more noticeable. The dash reads perfectly normal good temp, good oil pressure. The truck has a little vibration at idle (1000rpms) nothing serious but don't know what causes the vibration. I don't think the noise I'm hearing is hurting anything or will lead to something more serious that's my thoughts. Can anyone help me out and put there two senses in here.