We put in a new sink/tap and plastic piping in bathroom about a year ago. Ever since, the water from that tap smells. Have tried plugging the drain and it still smells. Doesn't smell in the kitchen or in the showers at all and never has - only in this newly replaced sink. The plumber said he thought it was something to do with the new aerators (not sure of that spelling) but we removed that and it didn't make a difference. He checked the trap and that is set up correctly. The only thing that is different in that sink than all the others in the house is that we took out the copper tubing and replaced it with the newer plastic/bendable type feed lines - wonder if THAT could be the problem? The water tastes fine and is perfectly clear but it's enough to make you gag while brushing your teeth - yuck!!!
Just a few suggestions; I find it hard to imagine that it really is the water that smells, so also check the following:
Most obvious is that sewer gas is coming in somehow. Check the p-trap for leaks, make sure there is always water in it. If you don't use the sink often, the p-trap water can evaporate.
Check the ventilation of the sewer lines. Could there be some obstruction in it? Assuming the smell started when the work was done, did anything else change in the system. Is it possible the vent was closed off or obstructed by mistake?
I'd guess that the issue is not the water but, instead, the flow through the drain. It makes zero sense that the water would get smelly from traveling through PEX.... unless the plumber re-used dirty PEX or something... but then just running the water for a while should rinse it out thoroughly.