Solved: The Reason Behind Your Sink's Strange Behavior During Toilet Flushes

replying to CAJ, Monte wrote: Sounds like a clogged drain and/or a clogged plumbing vent. Clogged vent stacks will occasionally occur cue to frost build up where it exits the roof in cold weather

replying to Monte, Jen wrote: So, do you know how we can unclog the vent?

I think you need to call a plumber

This issue may be caused by a problem with the air admittance valve (AAV) or the vent pipe in the plumbing system. The AAV is a device that is installed in the drain line of the sink, which allows air into the system to equalize pressure and prevent water from flowing back into the sink when the toilet is flushed. If the AAV is faulty or improperly installed, it can cause the sink to run when the toilet is flushed.

The vent pipe also plays a role in preventing this issue, it allows air into the drain lines to balance the pressure, if the vent pipe is blocked, not properly installed or have any leaks it may cause the same problem. It's best to have a plumber inspect and diagnose the issue to determine the cause and provide a solution.

It's possible for a DIYer to fix the issue of the sink running when the toilet is flushed, but it will depend on the cause of the problem. Some potential causes include a malfunctioning air admittance valve, a blocked vent stack, or a problem with the plumbing vent. A DIYer with some plumbing experience may be able to diagnose and fix the issue, but in some cases, it may be best to call a professional plumber to make sure the problem is fixed correctly. To fix the problem, it could be as simple as cleaning or replacing an air admittance valve or vent stack, or it could be more complex, such as rerouting the plumbing vents. However, to know for sure, it is best to call a professional plumber for an assessment of the situation.