50-ft saucer with 6 bright white lights shining down outside Grass Valley, California

It was the second week of November in 1986.  My husband and I were in the Sierra Nevada Mts between Lake Tahoe and Grass Valley California.  It was about midnight and we were walking on the top of a mountain when suddenly there came a noise that sounded somewhat like a very loud generator engine.  Doug and I discussed the possible origin of the noise, concluding that there were no buildings within two miles.
The noise became louder and louder as I looked into the sky to see a circular object just above the tree tops (probably 60-80 feet).   We never saw the top of the craft, but clearly saw six bright white circular lights shining from it's bottom surface and it was probably fifty feet in diameter .	The craft hovered above us for less than one minute and during that time Doug and I both ran downhill as quickly as we could.  I was not watching the craft as it departed, but did hear the noise end as quickly as it had started just about two to three minutes earlier.
Source of report indicates that the date of the incident is approximate.  PD