My 2000 Audi A6 2.7T has 328k kilometers (205k miles about) and it makes a weird, loud humming noise when the engine is running, and when you drive slowly. The noise can be described as an unhealthy sounding engine, it comes directly from the engine bay. Sounds sort of like a loud hum, and the check engine light is on. Any ideas?

 

My hum was a loud AC condensor fan. If I used ac, the fan would spin up and down depending if I was moving or not.