newby hi to all and novice at plumbing. i have a two story house with two baths, toilets above each other. when you flush the upstairs toilet the downstairs one bubbles almost instantly. i had the main line snaked out professionally and is clear. i cleaned out the vent at the roof and when i open the cleanout behind the downstairs toilet i can see some daylight in the pipe. the downstairs toilet it on the bottom floor in concrete, believe it is original to the house not added. there is a storage room behind the bathroom where the waste line comes from upstairs down, a cleanout, and then goes under the concrete i believe to conect with the toilet. the house was built in 1979. i have lived here two years and within the last year started hearing it but if you arent downstairs when someone flushes the upstairs toilet you wont hear it. please help…thanks