I have a new bathroom sink; pvc elbow and the lines in wall cleaned out before installed. It's only about two months old. All was fine but now, after the water goes down the drain, it starts to make gurgling, glub, glub sounds. The water flows swiftly down, however. Why now and should I do anything at all?

Your new lavatory. Is it a regular bowl with a overflow or is it a vessel sink without one? A gurgle tells you the lavatory's venting through the trap. So what do you have? Tom

Well, now I'm going to sound like a real idiot--I'm not sure--there's a hole in the front upper edge if that means anything and I know that, when you have to plunge the drain, you hold a towel or something over that hole. My curiosity is that this did not happen the first weeks the sink was installed; it's a new development yet the water flows freely.

You have a lavatory with a overflow that's venting through the trap. That's the gurgle that you hear. This is caused by a blocked vent.
The solution would be to snake the lavatory roof vent. Put out enough snake to reach the base and 20 feet more. Good luck, Tom