Maybe not so much a gurgle as a blub blub. It's a '95 m900. Had my tank rust coated a while ago (less than a year? I need to check my notes). Installed an electric fuel pump to replace the stock vacuum pump. I think that was close to the same time as the tank.
I'm not sure when it started, but I've noticed that when I pull into the garage after a long ride the tank will gurgle as I'm taking off my gear. Haven't tried opening the cap right away yet. Keep forgetting.
Thoughts?
Probably from the check valve in the filler cap, open the cap and see if the noise stops. As long as you are not developing vacuum in the tank this should not be a problem.