I am 95% sure it's your A/C compressor turning on and off. This is normal. If you're really worried about it, open the hood and see whether the plate on the front of the compressor spins when you hear the engine "lugging." It's on the left side of the engine and driven by the fan belt. You're looking at the end cap over the magnetic clutch that engages to drive the compressor when the ECM says it needs to provide pressure.
Alternatively, you can switch off the A/C and see if the sound goes away.
I just got a new compressor today, so I can verify that both a dying compressor and a brand new compressor will make the engine behave the same way at idle
Mine lugs a little more under the compressor load when it's warmed up.
(Incidentally, a dying compressor sounds like a vacuum cleaner whine and it may be louder than your engine when its running...)