When I bought my power-vent, the dealer included a rheostat to control the speed of the power-vent's blower/fan. It's just a dial that controls how much power gets passed to the power-vent, like the dial on a box-fan. Some are analog, some have high-med-low settings.
A quick search turned up this one
Mine's different, so I don't know if that one is good or bad or anything.
I have read/been told that they may not good for your power-vent unit, as they're designed to run at full throttle all the time, and it make cause extra stress to not be running at the designed speed. To change the draft, you're supposed to change other settings elsewhere, but those aren't going to help you with how loud things are.
That said, I have one on mine and use it, and I know several other people have said they have 'em too. You can always contact the manufacturer of your power-vent and ask them what they think the proper solution to the noise is.