I noticed a few days ago that, whenever my dishwasher is draining, the sink in my kitchen island gurgles (glug, glug, glug). I THINK that the sink connects to the main kitchen sink drain underneath the floor. Any idea what could be causing this? Also, I'm not sure if it's related but I just noticed that my upstairs bathtub is draining really slowly now. But I'm pretty sure it's a separate line until it gets to the Septic connection like everything else.
There's a blockage in the venting where the island ties-in. The 2 locations are commonly vented together, due to that loop under the island sink. A T-fitting instead of a Wye may have been used to tie-in the island and this is why only a Wye or directional fitting should only ever be used.
I think it's because your sink drain and your dishwasher drain are somewhere connected. It's the same in mine.