Hey,
I have a 1998 2.8 A4 with 100,xxx miles on it. it has been driving great, and then one day i noticed that when i revved the engine a little, it was making a noise similiar to a diesel truck. There is no whining or squeling, and it seems like the noise starts when the car is warm. It almost sounds like there is an industrialfan inside the engine. It seems to be driving ok, maybe a little lack of power. The noise is in any way extremely obvious,i can only hear it when the radio is down, or if i am in a parking lot.Some say that the fan is on "high" speed because of a broken temp gauge, others are saying that its a tensioner of some sort. Either way, i dont have the money to fix it now, so would either problem destroy the engine if i kept driving it? Thanks

where under the hood does it sound like its coming from

could it be an exhaust leak?!?

I'm going to give a +1 to an exhaust leak, probably flex connector.

Check your oil level.