My house was built in 1934 but has mostly been updated. We have one updated bath on the first floor and a new bath on the second floor. My concern is that when I flush the downstairs toilet there is a gurgling noise that seems to be coming from the bathtub. The toilet also has a little trouble flushing sometimes, it hasn't overflowed but it has come close. The upstairs bath seems to be working normally. Any suggestions or is this something to worry over?

My concern is that when I flush the downstairs toilet there is a gurgling noise that seems to be coming from the bathtub. The toilet also has a little trouble flushing sometimes, it hasn't overflowed but it has come close. The upstairs bath seems to be working normally. Any suggestions or is this something to worry over?
This tells me that your vent's blocked and the system's venting through the tub trap. The solution would be to rent a sewer machine, take it up on the roof and snake the lavatory vent. Put out enough cable to reach the base and 18 feet more to get into the main. Good luck, Tom

Yep, Tom's got this nailed. How steep is your roof and what material is your vent made from, cast or PVC?