Sometimes I notice a buzzing sound at around 5500-6K rpm when the A/C or heat is on. It's not particularly loud or very annoying, but it's noticable. The ride of the car or engine isn't affected by it, since it goes away when i turn the AC/heat off and go back into that rpm range. I assume this is simply a fan problem but I wanted to try and pinpoint it; anyone else ever had this. Weird...