The bathroom sink has a strong odor coming from it. The drain line seems to work well but there is always standing water in the overflow hole. We don't let the water in the sink get high enough for the overflow to have to take on the excess but somehow water always seems to be there. I've tried to soak up the water with a wick but then it will be back in a few days. I think the odor is coming from the overflow but am not sure. You can turn on the water in the sink and the smell hits you hard. The sink is at least 30 years old. I have put drain cleaner down the overflow hole and have also put down clorox. Any suggestions?

You can get mildew growing inside the overflow vent, it's a semi closed chamber nice and dark and wet. If you close the drain stopper and turn on the water do you get the same strong odor?

When I turn on the water with the drain plugged and let it fill into the sink, I do not smell anything.

It looks like your sink drain assembly is not for sinks with overflow ( no oveflow holes in the drain ). Water has no place to go and after few days will starts smelling...