If you look at the pump specs:
Exa-Pump®
You can see that it is rated to move up to 3 gallons of oil per minute. This is essentially an overkill for this set-up, but this is by far the best pump you can get for this application. Because the pump is moving a small fraction of it's maximum capacity (especially at idle), it will naturally make more noise. There is just not enough oil to fill the gear housing and silence the pump.
There is nothing to worry about, the pump is working fine.