Slight intermittent moaning noise when using A/C

I have a 2015 TRD Off Road 4x4 with 39k miles on it. Ever since I bought it a couple of thousand miles ago, when running the A/C, there's a slightly moaning noise coming from the engine compartment when off idle. It seems to occur around 900-1200 RPM when the compressor is engaged. At idle if I get out of the truck, I can't hear it. When cruising around normally, I don't hear it. It usually happens when decelerating for a stop and then it starts. As the engine RPMs change as the truck downshifts, you can hear it when it sweeps through the RPM range I mentioned above. It only does it when using the A/C, so I know it's isolated to something related to the A/C system. I've had a lot of Toyota's, along with four other Tacomas, and none made this noise. It's not very loud, but you can hear it inside of the cab.

The compressor sounds fine at idle and at higher RPMs (no groaning or grinding). The A/C also seems to work OK and on par with what I experienced in my 05 4Runner V8 Limited (nothing to write home about). I don't know if it's possibly the belt, idler or something going on with the compressor (or a resonance within the system). I know the clutches can give out, but I don't detect any squealing or burnt rubber smell, so I don't believe the clutch is the culprit at this point.

Anyone else experienced this?

 

Same thing happened in my corolla. It ended up being the clutch on the compressor. It turned from a hum to a grinding noise within a few months. You might want to try looking at the clutch, see if it has like any sort of rust or residue on it that could indicate a possible problem. What sucks the most if it is, if the Tacoma is like my car, the compressor will kick on and off if you have any fans on in your car at all. If it ends up getting worse, like if you start hearing pretty obvious grinding noises and the engine acts a bit limp quit using your fans until you can get it fixed. In the mean time, maybe make sure your freon levels are good and maybe have some dye put in to see if you have a leak. If the compressor is working too hard it can cause it to eventually fail. Never use A/C pro, shouldn't have to say that but it has an anti-leak component that will gunk up a compressor, which is what ultimately killed mine.
If you can't replace the clutch itself, DO NOT take it to a shop or dealership, my god, I got quotes to replace the compressor between 1k-2k and quotes to replace the whole system at 1.8k-4k. You can buy a compressor kit with all of your rings, the receiver dryer, evap valve (I didn't replace it, I think I would have had to move my passenger airbag or something stupid like that. It's just machined aluminum, it'll last forever) and the compressor, as well as hopefully the lubricant for it. The kit was about $200, to have my system drained of freon was $55, I drove it home and made sure (through both valves) that the system was completely depressurized (after driving the system can build a bit of pressure) We took out the compressor and receiver dryer and put those in (make sure you look up how much lubricant the compressor is supposed to have inside before putting it on). The compressor is a pain to wiggle out but not too bad. This is also a good time to replace your belt if you need to btw. Then I had my shop put the freon back in as well as some dye to make sure I didn't have any leaks to begin with, since I didn't replace the whole system; that was about $60. Took me about 5 hours total. If you get anything on your serpentine belt during the job it will squeak, at first I thought I didn't line the belt up right or something but I just put some lubricant on that and haven't had an issue since.
At your mileage I'm surprised you're having any issues though. My car was almost at 250k when the clutch went out, so I'd check any warranties you might have yadda yadda if you haven't. hope this helps