Thanks. I'm still running the info but don't have a good answer at this point.
The hiss tells us that a pressure differential is being equalized. It can be from vacuum inside the tank or pressure being released-- both are possible with evaporative purge flow and heat buildup.
If you wouldn't mind taking more time, put your nose next to the gas cap the next time you have an opportunity to twist the cap when the tank is gurgling. If you smell fuel odor, it's a pressure release. If no smell is noted, it's likely vacuum and air is being drawn into the tank.
The difference in possibilities is great. If the tank is being cavitated, you may be boiling fuel at low temperatures due to the loss of atmospheric pressure. A release of gas fumes from the cap suggests that the tank is being pressurized from heat, but I can't identify the source at this point if that's true. It's most likely a cavitation problem, but it should stop almost immediately when the cap is cracked. Boiling from heat would likely get worse with loss of cap pressure and I'd expect the noise to increase at that point.
So... no, I don't know the answer. If you wouldn't mind taking the sniff test when convenient, I'd be interested in hearing the results. It would at least identify whether the tank is operating at a high or low pressure, which may make it possible to diagnose.
It's a weird one, I'll give you that!