sounds like the venting part of the plumbing system is "starved" for air. There might be a clogged vent locally, in the wall piping or maybe as it is common for houses surrounded by trees at the vent terminal on the roof. Quite often leaves will clog up the 4", 3" or 2" pipe that is on your roof. When this opening or pipes in the wall are clogged, the negative pressure made by flushing a toilet or running water causes noise. Thats my best bet.
Questions: does it only occur in that bathroom?
if yes, then problem is possibly there