Overdrive is really just 4th gear. OD off prevents the trans from going into fourth gear and instead keeps it in 3rd, or direct drive. This is done to reduce head buildup and clutch/band wear when towing. As 3rd provides a better rpm range when traveling above 45ish mph compared to 4th.
The noise you're hearing is the engine being back driven by the transmission when the Torque converter is in lockup, which happens above 45ish mph. That noise is completely normal, that is about as close as an automatic will ever be to a manual transmission is when backdriving the engine.
Its completely harmless.
