[C320-list] Plasticky Smell From Head Area

[FSenft at aol.com]

Hi All:
We are experiencing an odd smell below deck in  our 3-year-old 320. Would 
love some advice. 
Hard to describe smells, but it is a plasticky/  burning odor, almost like 
fresh PVC. It seems to arrive from the head, but could  be from the aft closet 
-- or anything nearby. It has been going on for over a  year and is strongest 
when hatches have been shut for a while. Had  suggestions that could be battery 
(no, replaced and oft filled) the head  treatment (no, several brands tried, 
incl eco bleach) or engine (doubtful, wrong  smell).  
Any suggestions would be great. Tough to  invite guests over with this going 
on.
Cheers,
Jon