Dave is right. Either the sink is unvented, or the vent is clogged. Many times the vent gets clogged from debris right at the top of the tie in for the drain at the wall. This happens when the drain is partially filled with sludge. Every time you run water down the drain, it can not go fast enough because of the sludge, so part of the water goes up the vent line and deposits soap scum and or hair on the inside of the vent. Eventually it cloggs up. Another thing that can happen on a galvanized pipe, is the inside of the line can flake off and become lodged on the horizontal portion of the vent that ties into the main vent from the toilet. When this happens, it blocks the flow of air. Once it becomes completely blocked, then the whirlpool thats created in the trap when you let out the water, continues to try to draw air once it has begun to run down the drain below the floor. A positive pressure is created when water enters the drain ahead of the water, and behind the water a negative (vacuum) is created.
Another factor is, do you have a vent on the drain at all? If the drain goes out of the sink and runs into a trap, but then goes straight down into the floor you do not have a trap at all. But, if the drain goes straight back from the trap into the wall, you may have a vent. Pull the trap off the wall, and look into the opening, if there is a tee in the wall, with one pipe going up and one down, then you have a vent. If the pipe simply turns down, there is no vent. If you do not have a vent, you will always get the gurgling.