Are the ducts outside of the conditioned space? attic? crawlspace?
When you have a leak in a supply duct that is outside the conditioned home you are piping air that would normally be in the home outside of it cause a negative pressure in the home to pull in debris/dust in the cracks and crevices and yes even construction debris from many many moons ago when the house was built.....it may have just sprung the leak bring that stuff into the home.
If your ducts are in the attic check to see if they are sealed....and not with tape (what happens to adhesive when it's really really hot like in an attic?)
Look at this .pdf and notice page 5 "common air leaks". Now imagine if your duct work in the attic sprung a leak and pressurized the attic space while putting the conditioned space in a negative pressure. All the arrows that are points losses in an upward direction would be pulling the opposite way and bring hot attic air and debris into the home if all those leaks are not sealed up like in most home.
There is a requirement for air exchange for conditioned space to purge indoor pollutants. It's 25% of the total volume of the home in one hour.
Let's just say for example you have a 3,000 sq ft house with 8ft cielings.
24,000 cubic feet of air.
If your duct's leak 10% on a 3 ton HVAC system that is moving the amount of air it should to work properly it would leak:
400 cubic feet of air per minute per ton = So the system move 1200 CFM
1200 CFM x 10% for duct leakeage is 120 CFM.
120 CFM x 60 to convert to hourly loss = 7200 CF of air or 33% exchange rate with just the system alone.....that doesn't take into account any other items that may be adding the that exchange rate like bath exhaust or range hood exhaust/vent....doors opening....ect...ect...
Same can be said if you have a return plenum/duct run that is in the attic with a leak it will suck attic air into the return stream and force it all the way through the entire home through the duct work.....just the return and supply change jobs.
No offense but I'm not sure I would want to add any more restriction the the return side without verifying it can take it. The recommended velocity for noise on return ducts is 300-400 feet per minute at 500 fpm it sounds like a vacuum and much higher produces whistling and sucking in the filter or moisture being pulled from the evap. coil onto insulation or other items near the coil.
When designing a return you have a factor called A/K that is supplied from the grille mfg. This accounts for effective area of a grille or filter grille. A/K factor is basically sq ft but it is of effective area and not total area meaning it only accounts for the small openings in the grille.
12 x 20 grill....looking at total A/K is 1.666 sq ft....actual effective for it from the mfg is .92 sq ft.....so why is this important?
using that effective area and a fpm requirement of 300 that grille can only support 276/cfm...simply done by multiplying fpm by the A/K factor to get a volume or cubic feet.
With two of those return grilles you can move 276/cfm x 2 or 552 cfm @ 300 fpm and minimal noise (probably not even noticeable) That's only enough for 1.5 ton unit......
For a 3 ton system you need 400cfm/ton or 1200 cfm remember. if you want your system to be as quiet as possible you would need
4 sq ft worth of effective area. Looking on my specification sheet for the return grille you would need 2-20" x 20" grilles...which is slightly less than 4 sq ft......My point is almost none of the systems you ever find are designed this way. They almost always push the 400-500 fpm which cause the noise issues mentioned and leave almost no room when the filter starts to get clogged.....CFM drops...system effectiveness drops...air feels colder coming out of vents
.........power bill rises......eventually coil freezes and causes a problem.
When it's designed for 300 fpm it gives you the air you need and removes the noise you don't. If you like those pleated filters or real thick media filters you must account for them in the design of the return grill by making it larges because of the large pressure drop they cause.
Sorry for the long post kinda hard to explain anything without some of the basics.
Hart & Cooley - Products - Residential & Commercial Grilles, Registers & Diffusers;* B-Vent Gas Vents, All-Fuel Chimney and Chimney Liners; Flexible Ducts
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Slight buildup in the duct work is not harmful as long as it doesn't get out of hand.
Are them ducts really clean.......????
