1. Does this "over" mean "He shouted louder than the motor" or "along with the motor sound"?
2. Does "thrumming" sound mean "low beating sound"? I can't think of it exactly.

st81)Before inviting Mr. Harton in, Bernard vacuummed.....He shouted the story of Mr. Harton over the small, thrumming vacuum motor, and Fern listened intently, trying to dodge the vacuum's zipping nose.