I have two Emperor 400's on my big tanks, I've been through three. My first purchase had one that was virtually silent and one that was noisy as heck and leaked at the bio-wheel spray bars. I returned the noisy/leaky one then purchased a E400 from a different vendor. This one wasn't as noisy, but still made enough noise to be noticable especially when compared to the first E400.
I tried reseating the impellor(?) a couple of times until I was happy as I was going to get.
Personally, I think many of the parts on the unit are kinda cheap considering the cost of the product.
I have a Penguin as well, zero problems and no noise with that one.