Keep an ear to the toilet next time you pump out
If you hear any noise that indicates air being pulled through it, your vent isn't allowing enough air into the tank. The two most common places for a vent blockage are the vent thru-hull and the connection to the tank--both the tank fitting and that end of the vent line. If your vent thru-hull is one that won't allow you to back flush the vent line with a garden hose, scrape out the thru-hull with a screwdriver blade or whatever works. If you still hear any hissing or whistling in the toilet, remove the vent line from the tank and scrape out that end of the hose and the fitting.
Improving the vent seems like it might be a good winter project this year.