I found that a lot of the noise is from the filter.
We have a high efficiency filter at the unit in the basement. It's kinda expensive at ~$100. To extend its life, I added a washable, very inefficient filter at the return in the great room, (w/ the HT). I figured the washable inefficient filter would catch the big stuff, not add much pressure, and give some extra months between replacement of the good filter in the basement.
It worked just like I thought. BUT, it was fairly loud when the system came on. And as the washable filter got dirty, it got louder. In fact, the level of its noise when I reared back for a nap was my indicator when to clean it. We did actually have to increase the HT volume at normal to softer listening levels.
Eventually it became enough of a distraction that I just took out the washable filter and only use the good one at the unit. Short of installing a filter at the unit, I suggest cleaning/changing the filter in your HT-room return. It will make a difference.