Hello,
My first floor toilet starting making soap suds while I was showering in the second floor bath directly above it. Recently I have noticed that I've had to plunge the first floor toilet more frequently. Do I have a sewer blockage or could this be a venting problem?
Thank you,
Mike

It could be either. Shower water is backing up into the toilet and a parytially clogged drain or vent can both cause this. Are there clean outs in the drain after the toilet connects to your main drain line?