Electrical static noise

Purchased a 1998 SLS jimmy 4x4 in August. Recently have been getting a static noise coming through the speakers. Only happens while running generally at idle and certain rev points. Much more pronounced when it is damp out. Though plug wires were going bad so replaced them and now almost seems worse( could be my imagination though). tried cleaning engine to firewall ground @ firewall but didn't help. I don't know where else there are grounds or if it might be something else. I bought the vehicle with 42000 and now has almost 50500. Any help would be greatly appreciated.