vibrating noise under the seat

What is your question?

Over 55 MPH a vibrating knocking sound is heard. Not very loud, yet varies in decimal level. Fixed the wheel wheel covers, checked the heat shield, checked the exhaust and no indication that noise would develop. Nothing loose under seat. Does not make any noise unless over 55 mph or higher.

What makes this problem better or worse?

speed abover 55 mph

How long has your 2014 Nissan Rogue had this problem?

5 or 6 months