Pull the dishwasher out enough so that you can look underneath it while it's running. If necessary, disconnect the water line and discharge hose and then reconnect them outside of the cabinet space temporarily so that you can get enough clearance to look underneath it while it's running. Look - don't touch anything!!! Moving parts, water and electricity!!! See if there's a metal bracket or clamp that's rubbing against the drive. Then, with the unit turned off AND unplugged from the outlet, look at the motor, and try to manually turn or move it. See if anything is binding or scraping.
Lastly, your dishwasher's sprayer arms may have a central telescopic water jet nozzle that extends upward to the bottom of the top rack when the arm is spinning. That may be rattling around as well.
Or, you might just have a very noisy FREE dishwasher.