actually, after noticing your observation about the leak sound, I would check the purge valve instead. There's a vacuum line that's connected to it, perhaps it has come loose. A disconnected vacuum line would also explain the erratic behavior off idle. The canister itself is basically a filter with charcoal, so there's not much that can fail on it. Both are related to fuel tank venting though, so it could be either.
In any case, it sounds like a problem with a your evaporative emissions control. it's not a critical problem, you can still drive around with it.
