Basically, it is down to how much the air is disturbed. Pellets have a more complex shape than slugs, involving the air getting around more changes in direction, so you would expect them to generally be more noisy. However, hollow points are going to generate some noise, so large hollow point slugs could also be noisy. As both slugs and pellets have rifling marks, I cannot see this as being a major difference between them. Pellets such as the JSB Hades can be expected to produce more noise, or anything which has cuts or marks on the nose to disturb the airflow.
As for reducing the noise, smooth boat tailed slugs with no meplat on the nose fired at relatively low speeds are probably as low as you are likely to get. An accurate round is likely to be less noisy than a round which is yawing, which will itself disturb the airflow. As for .177 being quieter than larger calibres, this may be true, but if the airflow remains laminar along the entire length there will be more base drag and turbulence which may cause a problem. It will all be down to the individual designs.
