A noisy return is usually a sign the duct work is too small to handle the required air flow. The fix is either increasing the size of the current return or adding a second return. Sharp turns in the duct work can also increase the noise level. Air flow can be decreased by in general it should be between 350 and 400 CFM for each ton of cooling.

Agree with Mike.

Usually the problem you describe is related to ductwork design/sizing and location of air handler or furnace. Dealer should be called back to evaluate and make recommendation to solve this problem.

IMO

Thanks, Mike. When you say "the duct work is too small to handle the required air flow" do you mean the return duct or the supplies, too? Is the AH putting out more CFMs than it is taking back in? There are lots of bends in the supplies. The flex from the main trunk twists and turns around itself in the attic. The installer said that's necessary to REDUCE noise. But, again, the noise is not at the supplies but at the return. In adding a second return, would it work if we added another vent opening in the same return duct, above the existing one without modifying the return duct itself or should the existing duct be enlarged? Should it be an additional duct from another location? Thanks, again.

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

Our return air ducts whistled loudly too, but our TRANE installer came back immediately to check the problem out. I told him it only occurred during the stage one heating, not the stage 2. When he pulled the filter out just a bit, the whistling stopped. He taped corners and plugged little holes in our old return air flow duct work. We keep the filter pulled out about 1/4 inch. Now the whistling is very slight.

"Is it a filter grill? If so does it whistle when the grill is open?"

O/P: check this - Inexpensive metal grills can "sing" like crazy if the airflow is high. A better quality grill may reduce this. Was the grill changed? What is the airflow setting on the blower?