I'm on a Mid 2010 Macbook Pro and the hissing does not appear in the rendered file or timeline; only in the exported vid. If anyone has a definitive fix I'd love to hear it. The hissing glitch appears for 2 seconds at the beginning of the file then dies down then reappears for two or three seconds after each part of the time line has been edited. EG removing some of the footage in order to replace with a title. I would love to har if anyone ut there can fix this. PS the denoiser tool was used to reduce ambient hiss on the raw footage but again no hissing appeared in the time line or in the rendered file. Only after export.
Do a forum search for Hiss
This is a common problem using certain audio effects in PP. The best solution at the moment is not to use those effects. Do that kind of work over in Audition.