I have a client who has a problem with a gurgling noise in her bathroom sink when ever the A/C is turned on. The condensation drain is connected from the A/C to the sink’s drain. What can i suggest to her as a solution? Thanks
Suggest get used to the gurgling. 
She can try cleaning out the trap, but I doubt it will help.
Unless you are married to this woman it ain’t your problem.
or she could have the drain line moved to another location…
Remove the gravity condensation line and install a condensation pump and relocate where the system drains.
Then she’ll complain about pump noise. Most likely there is no drain trap and the drain is on the negative pressure side of the blower. Make sure it has a properly installed trap.
It sounds to me like she already has a condensation pump, and that is causing the noise.
She could also as another suggestion stated, have another drain installed.
If she already has a condensate pump then the pump check valve is dirty and needs to be removed and cleaned. In fact the condensate pump should be cleaned annually. Period.
Could be that the trap seal (if there is a trap) might not be deep enough to resist the positive air pressures developed by the blower. Air is being intermittently blown out the line causing the gurgling.
Perhaps the condensate line could be routed to the outside of the house instead of into a drain.
In this area, condensate drain lines are not allowed to be routed into the house drain system.
Wait though, you’re in Canada, don’t you guys only use the A/C in the summer…all three weeks when it’s 75 degrees? 
Thanks Guy’s the information is helpful i will investigate further and let you know what works.
My guess is there is no P-trap at the unit and the blower is straight piped to the sink and is trying to pressure the sink trap thus the noise. Put a trap at the unit and at least in the summer time there will be no noise.
One can use a petroleum base liquid in the trap at the unit during the winter and not have the noise then. Here are some pics of what I see a lot of totally wrong and I write everyone I run across unless the furnace is high efficiency.
The second pic is where the condensate drains into the stack 2 inch vent and the 3rd pic is where the 2 inch ties into a 3 inch stack wants to ensure there was plenty of sewer gas to spread around for all.
That was my assessment as i didn’t see a trap. Thank you Charley for your informationi will recommend the installation of a trap to eliminate the noise