How to Stop Ductwork Popping Noise when AC or Furnace Turns On and Off

So I recently started hearing my HVAC Ductwork noticeably making a loud popping sound when my Air Conditioner started running, and then it would make the popping noise after the AC turned off (but no popping noise while it was running). In my case, it was really loud when the daytime temperature was over 90 degrees with a lot of humidity, but I believe it was doing it in the winter as well, just not as loud. I believe the popping noises may be louder when temperatures are at extremes (either really High when AC is running, or really low when Furnace is running). Apparently, this popping noise that the ductwork makes is called 'Oil Canning', which is due to air pressure changes in the ductwork, typically causing the rectangular (or square) metal duct to flex and relax, which can cause the popping noises in your air ducts at startup or shut off. To help prevent this from happening, you should see a scribed 'X' on the two large sides of rectangular ducts (rather than being totally flat on all four sides), the 'X' can add a little structure and help prevent the ducts from popping (but it doesn't always work).

Keep in mind, if you're only hearing the 'Oil Canning' popping sound on startup / shut off of your Air Conditioner or Furnace, then it's probably nothing too serious to worry about, and more just an annoying sound. However, if the popping sound is continuous, then you may want to call an HVAC professional to get things sorted out.

With that said, here are the steps I took to fix my ductwork from Popping when my Air Conditioner was turned on, and turned off:

1. The first thing I did was locate the origin of the popping noise.  Most likely this oil canning noise is coming from your larger Return air ducting, near (or at your furnace). In my case I knew the sound was coming from down in the basement near my furnace but wasn't sure of the exact location in the ductwork. So after watching the back of the Return air duct when my AC turned on and off, I could visually see the large rectangular return duct get sucked in (making a pop noise) after the AC started running, then after the AC stopped running the duct would be pushed back out (making and even louder popping noise two times). This was happening on the large vertical return duct approximately 35" tall x 24" wide x 12" deep, a few feet away from the HVAC return filter.
2. Once you locate the ductwork that is popping in / out, you'll want to reinforce the duct with wood or metal strapping. I've seen some people mention using ductwork S Clips / S Cleats as the cross brace, which can be bent around your return duct. But in my case, I used some cheap light weight wood furring strip strapping (3/4" x 1 1/2") that I already had at my house. I cut a 25 1/2" piece, then two pieces (12"), so that it could wrap snuggly around the three sides of the duct in a flattend U shape. (If needed, you could box in the duct on all four sides, but for simplicity, I just made the strapping to slide on to three sides of the return duct).
3. I then drilled 2 holes on either side of the 25 1/2" piece of strapping, so I could then screw in the two side pieces of strapping with four 1 1/4" screws.  In my case, I had to cut down one of the side pieces, to better accommodate the side of my Return duct, but if you can get away with it, it's best to keep your two side straps and the full length (depth) of your duct.
4. Next slide the the wood strapping that you just built over the return duct that was popping. I'd recommend putting it in the center (where 'X' might be). In my case, I couldn't quite putting the strapping in the center due to my Humidifier control panel being in the way, which may have been why I needed to use two separate reinforcements over the ducting. My strapping attached around the return duct snuggly enough, that it stayed in place without any tape or screws.  I personally didn't want to put any screw holes in the return duct. 
5. I then waited for my Air Conditioner to stop running (or you could just turn off the Switch to your furnace / AC), and for the return duct to pop out (not sucked in). Then I used HVAC foil tape, to tape the return duct, to the wood strapping. I think taping from the bottom to the top worked best, just make sure you don't push in the return duct, and that the foil tape is snug around the strapping.
6. After tapping up the wood strapping to reinforce the return duct, the Air Conditioner turned back on, and I no longer had any popping noise when the AC began to run.  However, the return duct still popped a little bit when the AC stopped running (but not as loud). So in my case, I added a second reinforcement of wood strapping a little higher up.
7. After adding the second wood strapping reinforcement around the duct, the return duct no longer was moving in or out, nor did it make any more popping noises when the AC turned on or turned off.
8. That's it! No more annoying HVAC ductwork Popping Noises!

I hope you're able to quiet your popping ductwork with this simple reinforcement around the return duct.

Also, if you haven't changed your filter to your furnace in a while, you may want to do that as well, especially if it's dirty.

On a separate note, if you hear a whistling noise throughout your house while the furnace / AC is running, and it's constant, it may be due to dampers being closed causing just enough air to flow through them to make a whistling noise. In the past, I made the mistake of closing a couple dampers in my basement (zone 2), and it created a whistling noise that was super annoying. I had thought it was due to my HVAC company, because they had just serviced my furnace and had them come back out to figure out the whistle (which they weren't able to figure out). Soon after I remembered I had  just closed two dampers, which then created the whistling noise throughout the house.

Lastly, just a reminder, that while you're messing around your furnace, it's a good idea to use some HVAC foil tape to seal up any air leaks around the ductwork that potentially is blowing out cold AC air or Hot furnace air into your basement, thus wasting energy and costing you money. Any connection points that aren't taped up with foil tape, can be losing air / air pressure. Use your hand to feel for air leaks around your furnace (the top of the furnace / ductwork connection points seem to be particularly leaky, since they almost never get taped / sealed up by the home builder).