Re: air bubbles
It might be a venting issue tough I would suspect more like a partial blockage of the drain pipe.
Usually a vent issue would cause a gurgle sound from the sink or tub when the toilet is flushed as it will try and draw air from wherever it can.
A simple test would be to disconnect the trap from the sink .... flush the toilet to see if the situation improves. What this would do is provide an open direct vent.
As mentioned earlier I suspect a blockage is likely the culprit since this has just occured recently.
What's happening is when the toilet is flushed there is a volume of air in the drain sending a bubble of air ahead of the discharge. When this hits a blockage this air bubble bounces back up into the toilet .
If the toilet is showing any signs of slow draining would be a clear indication of a larger blockage .... perhaps something lodged in the toilet siphon trap or something further down the line.
Which may be resolved with a toilet auger or the removal of the toilet and fishing out the obstruction .... or .... snaking out the drain line.
Hope this helps.
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