Purple lights on a low- and slow-flying, hissing object.

A low-flying object flew over the road I was walking down.  It made a soft hissing sound  which is what made me look up.  I could see some purple lights on the underside (only 2 or 3 but my view was obstructed by trees on the sides of the road).  It was a very dark night so I had a hard time discerning size and height but I estimated it was about 50-60 feet up and 80 feet across or more.  It was moving at only 5-10 mph.  The purple lights were a dark color and not terribly bright.  I saw no other color lights on the object (ie, no normal white/red/green aircraft lighting).  (It was not a hot-air balloon, I know what they sound like, plus I would have seen it glowing in the darkness).  I didn't feel anything (breeze, warmth, static, etc) from the object. I ran up to the intersection and crossed the street to try to see it after it passed over the trees, but I was unable to see it. I was in college at the time and had not been drinking.
Witness indicates that date of incident is approximate.  PD