Central air unit air flow noise problem

I have a 5 ton TRAIN heating/cooling unit. The duct work in my 32 year old house is steel. When the system comes on, the noise from the air flow is so loud that we can't hear the TV unless we increase the volume. Besides, its very annoying. (All the registers are open)
I had a AC service company check it out and he said there is not enough air flow from the intake. So he added a bid 18X18 vent and a large flex tube to go with it. It did not help at all. I replaced the duct on one of the registers with flex duct and it is quiter.
My question is: does replacing the extension ducts that branch off of the main duct with flex duct solve my problem or I am gonna do all that work and discover that there is other issues?
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Parse wrote:

There are a couple of possibles that occur to me - a non HVAC person. Are the ducts sized to accept the air flow? There are ASHRAE manual calculations for duct sizing. Are the metal ducts not insulated or mounted in a way that emphasizes the noise? TB
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Looking back over the ACCA's Manual D, the chapter on "noise" states "Noise is created when turbulence is generated in a duct run. This turbulence is usually associated with inefficient fittings (elbows, tees, transitions and takeoffs) and sternly throttled dampers." ..... "Regardless of the source, generated noise will propagate downstream through the supply duct and upstream of the return duct." .... "will enter a room via the supply ducts" ... "can be transmitted through the duct walls to the surrounding space."
Another place that subject to noise is the registers themselves. "Noise is generated when air flows through a grill, register or diffuser The intensity of this noise depends on the velocity of the flow" ... "Normally a supply outlet will not generate objectionable noise if the face velocity is less than 700 FPM. (Return air should be held to 500 FPM or less.)
Basically, if the duct is to small, that's a real problem. If it's fittings, you could try listening to each turn or takeoff starting at the furnace and try to isolate which ones are the real troublemakers. (If you do find specific duct problems, you will probably need to have the HVAC company back to make the changes.
My first assumption would be that duct work is sized to small. (Five ton is a rather large unit). And if that is the problem, it's not heating nor (especially) cooling properly as well. (Did you have the Trane installed recently? If so, you may need to have additional ducts installed to lower the velocity to acceptable levels; quieting the system and moving more air.)

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One possibility you might check is the blower in the furnace. If the original design used a blower driven by a v-belt but was later replaced with one with an internal motor, noise can increase markedly.
Hul

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