Doesn't happen after using a bunch of water. I'm Gone throughout the day, and when I come home i hear it making this noise after hours of not being used. The tank water level is normal - I've played with the flapper and it doesn't help.
More background:
Moved in 3 months ago. About a month ago, I noticed that after being flushed, the bowl water quickly drained to practically none maybe 10 mins after flushing. As the draining would stop, a weird noise would come from the bowl. Then it would stop. Then, as I used the RR and the toilet would have urine flowing in (sorry for being graphic), the weird noise would happen again.
It has since stopped making THAT weird noise, and the draining happens at a much slower rate. But now, after hours of not being used, it starts making the second weird, groaning noise with no movement in the bowl water or tank water (ie: bubbles, gurgling, etc).