The left hand side (as seated in car) is air intake for engine, right hand side is an engine bay vent. Anyhow, sounds like the clips on bottom of panel are not engaged. Here's a link to a common mod on the 986s (Boxsters from 1997 thru 2004) called de-snorking (improves the sound of the motor). Details on how vent panel is properly attached included:
10min. Boxster "de-snorkel" Hack! :: Car & Lifestyle Forum
Good luck