Take a peek at these sites listed. It may give you some insight on
your problem.

1) http://www.halifax.ca/hrwc/ThermalEx...Awareness.html

2) http://www.leesburgva.gov/services/u...og_thermal.pdf

Do a search on Google for more info........"thermal water heater
expansion tank toilet"

But it sounds to me that either.........
1) the heater may be set to a very high temp and causing thermal
expansion that your new expansion tank cant handle ( check setting of
your water heater temp , set for between 115-125 degrees.

2) Your toilet could have its own expansion relief valve. If not, then
this is one of the signs ( hissing ) of thermal expansion.

Either way, thermal expansion is too high and must be dealt with.

Dean