99 xj 4.0 with AC. Every time the rpms are around 800-1200 it makes like a grinding, vibrating sound. I thought it was the PS pump so I switched it out for an autozone one cause the bearing felt loose...wrong. replaced both idlers...wrong. checked U joint but I am certain it is rpm based. All the other pullies feel tight and don't make noise when I spin them. I can't for the life of me figure it out but it is driving me crazy. Belt tension feels good and everything looks to be right. Maybe my oil pump is dying? Great pressure though, always at least 60 lbs.....help?