You have an unbalanced duct system, or the sheetmetal is too light of a guage. The bang is the duct presuring up. I would imagine the static pressure is too high. Make sure the fan is not set too high of a speed, keep the filter clean. You could have too much pressure from a oversized filter. The return may be too small for the supply side. Get a tech to find the problem.

You stated the return is making the noise, but the main supply trunk may also me too small and contributing to the noise. Even if the ducts are not noisy, they may be too restrictive. This increases the static pressure and causes the air handler to work harder and eventually result in premature failure. The air handler can only put out what it takes in. Typically there is one return but several branches to the main supply. It thought you had hard ducts. The flexible duct can twist but not to the point where it is impeding air flow. I don't see how twisting makes the duct quieter. If that is the case, then see if you can make the noise worse by untwisting it. Take a look at Chart 4 at the link below. It says a 4 ton air handler needs a 16 inch and a 14 inch flexible return duct. I would want to see a second duct return added. Here is a link that might be useful: Rules and Rules of Thumb for Duct Systems

We had to go out of town so no pics yet. I will do it later next week when we get back. We are using pleated filters now that are so expensive although they are not the most expensive . They are at Lowes and are 2/$20. I hate paying that much but thought it was to clean MY air not the air for the mechanics...although it makes sense that it does it for both but more for the mechanics. Since I am seeing the "black/charcoal" color show up on the filters so soon after replacement I am wondering if I should have the duct work cleaned/whole system. It is only a couple years old though and I had them come and clean the squirrel cage etc 1 1/2 yrs ago. He said it was dirty. But we have not had the duct work cleaned...I always read conflicting reports as to whether to do it or not. We have a certified person that advertises in the area and has been in business in HVAC for many years. Opinion on that? As to the noise I gave up the other night and we moved to another bedroom...complete silence...ahhh...so pleasant. You can't hear even the air flow in the floor vents, it was wonderful. So that is an option for now is move out. I will get pics and look at the other filters and also would appreciate a link to any other kinds of return covers...I tried a Google search and didn't see anything that said it was open and less restrictive to air flow. Thank you for all the suggestions...I am going to keep on this. c

If a new grill has the same exterior dimensions, won't the old holes will be covered if the new one's screw holes are in different locations? These things are usually screwed in with small diameter self-tapping screws that are easy to screw in so it really isn't necessary to hit the old holes or to make new guide holes first. Or do you have a different concern in mind?