Our unvented system is quieter than our old vented one and we didn't have any pumps! The noise we had, especially in the night, was when someone went to the bathroom and flushed the loo. The noise we had was of the cold water storage tank in the loft filling up. Oh, and we have a more powerful shower now too.
OP. You'd be wise to get your water flow and pressure checked first - if it's not enough, an unvented system will be a waste of money for you.