I've recently noticed, perhaps because my windows are open in the Springtime air, that my 2000 323i makes a little honking noise upon acceleration. The only time it's noticeable when driving is when I first take off from a stop. honk.
I also noticed a little honk when I stated it, and a little honk when I shut it down, as the RPM's dropped from idle to zero.
It's running fine, as far as I can tell...