I agree that any prop can be noisy - just like any jet. It can depend upon the age of the machine and where you are seated.
In Props, the main thing to avoid is being in the row that is 'in line' with the props themselves. I tend to get to the back. Ear plugs are a standard part of my travel kit but normally used for anti-baby defence!
Also, I invested in a pair of noise cancelling headphones for my mini-disk player. These are useful even if you don't play anything through them!!! They are not cheap but very good for plans and trains.