My furnace is 11years old and i can hear water trickling inside the exhaust pipe. The vent pipe goes up through the attic.This is the first time i've ever heard this. thanks

Check the condensate drain on the furnace. Check to make sure the discharge pipe is clear of all debris.

It is normal for condensation with a 90% furnace. If the unit has a problem draining water the pressure switch will open and the furnace will shut down and not work at all.