We have been having an issue when its really cold outside, with low water levels in toilets and a bad small coming from the laundry room sink (only when the low toilet level happens). The laundry room is on the main floor across from the main bathroom. We have a bungalow (built in 2010), and are on well and septic. Our tank was pumped summer 2012. The low water level in the toilet bowls is corrected with a flush but then sometime while we aren't watching it goes down again. I have noticed when I flush to fix the low level, there is a glugging noise in the laundry room sink pipes and the water splashes up from the toilet. I run water down the laundry room sink which fixes the smell for a bit. Could this be a venting issue or a septic issue? Thoughts?

Thankyou!
Melanie