Septic System odor

We've lived in this house in the country for a little over three years...it is almost 20 years old. Just for peace of mind I had my septic tank pumped out about 6 months ago. When they took the cover off, it looked like it was about two thirds full...my estimate after seeing it empty. No real problems since we've been here except a saturated area on the downhill from our drain field when it rains heavily for long periods of time. However, it happens to be raining today and upon entering the downstairs bathroom, I noticed a very unpleasant odor (methane?). It didn't seem to last too long, but it is the first time I've ever noticed this odor. Have I got a venting issue or worse?

2/3 full is about right, If thats liquid. What counts is how much solids on the bottom, which should not get over 18" or so. Without a disposal which is septic death, I go about 15 years easy and get about 12" solids build up... I think the septic guys say 5 years which might be right for sand filters on the outlet and with lotsa kids and disposals. When the odor is regular, get worried.