HKS Legamax Exhaust is for sale for your stock e90 335i exhaust (N54 or N55 as it doesn't matter) + cash. This exhaust has a very smooth tone and doesn't drone compared to other aftermarket exhausts. It growls when you floor the pedal and when you lightly step on the gas, the exhaust is pretty quiet. Again no drone when cruising on the freeway at 70mph as you can listen to music and conversate without yelling. I have a Berk 4" catless downpipe paired with it as it sounds amazing.
Here's a sound clip when I was making it growl on the GoPro Hero 2.
A couple revs at start up