Air Conditioning - Air Filter Whistle

I recently had the evaporator coils in my home A/C unit cleaned. Now that the air flow through the system has been improved, the air filter makes a whistling noise. I have tried three different types of air filters, but the whistling noise will not go away. I am guessing that the root cause is the system is pulling more air at a higher velocity through the filter. Any suggestions on eliminating the whistling noise?

Have you tried the cheap disposable ones you get from the drug store? They are very permeable and let a lot of air through easily. I have to use those in order to avoid a hissing noise.

How many tons is the system? What size is the filter? It sounds like (no pun intended) you need a larger return air filter vent and possibly a larger return duct.

I have a 3 ton system with a 16x20x1 filter.

Your diagnosis is right on. Your return air grill/filter is too small. Generally 1 square foot per ton is considered minimum. A 16x20 return air filter equals 2.22 sq/ft. You need at least 3 sq/ft. I usually go with 20x25 for a 3 ton, which is about 3.5 sq/ft.

It would be difficult to say how much work is involved. Have a good tech look at it. Also, the return air duct needs to be looked at. It may be too small as well.

Did the system ever freeze-up? It's a real possibility with restricted air flow.

The system has never frozen up. I looked in the installation guide and found that the motor speed can be adjusted. Do you think this would fix the problem? Should I leave this adjustment to a professional?

At this point I would not change the blower speed for a couple reasons. While slowing down the blower will reduce noise, it will compound the problem of not enough air flowing across the coil. Generally, you need about 400 cfm of air for each ton of air conditioning. So 1200 cfm would be needed in your case. Reduced air flow, whether it be from incorrect ducts or a blower set too low will increase the likelyhood of freeze-up and reduced system efficiency. It is nearly impossible to get the correct freon charge with poor air flow over the coil. In such cases the system will appear to be undercharged (due to low suction pressure) but in fact may be overcharged. In any event, this places the compressor in some danger. Short compressor life is common when the system is starved for air.

This sounds like a builder install, where cost is more important than doing it right. Another possibility is the house was designed for a 2 or 2 1/2 ton system, and the builder got a deal on 3 ton equipment and didn't do the duct upgrade. I can only speculate based on what I have seen in the past.

I believe your priority should be getting the correct air flow.

With the blower running there. Take and see how hard it is to open the blower door there . what is the size of the cold air drop to the furnace and how many cold air returns do you have there in the home.