Well I went back to the BMW dealership and test drove another E60 with ARS and it was making the same noise, just a little quieter. So I guess itís kind of normal. Or all three cars have the same problem.
I have the car back and am going to investigate other causes for the noise. If I can pinpoint it, the dealer I bought it from (non-BMW) said they would still fix it.
I think I am going to start by pulling the ARS fuse and seeing if it still makes the noise. Also, the area the sound is coming from is right where the seat belt tensioner is. I will let you know if that ends up being the culprit.