Sounds like you are referring to the black screw-in Air Admittance Valves located after the "P" traps. These are used to prevent vacuum in drain lines so the 'P' traps don't get emptied when you dump the tanks.
The valves could be sticking, I guess one could unscrew them and try soaking them, but they are fairly inexpensive item to replace.
Also, check that your roof stink pipes aren't blocked.
Hope this helps.