Just a few random thoughts:
1) If you have the space in the return, the "egg crate" sound asorbing
insulation used in recording studios might make a difference.
2) Also, since there is a garage below you, will your homeowners association
allow you to run 8 x 18 return trunk in the garage to a suitable closet?
Never hurts to ask.
3) Switch to a 95% AFUE furnace - you won't believe how quiet sealed
combustion is. Hopefully the existing venting will allow a double wall pipe,
if not, Tranes furnace is rated for downflow discharge.
4) If your water heater is in the same utility area, can you go to a tankless
water heater and use the space to move the returm
5) change the supply ductwork - add a few supplies sudewall from the utility
area = bigger ductwork = less noise
#5 Move is not an option at this time. Jeesh, I'm waiting for the market to improve.
I'm just going to have to settle with using neutral odor duct/plenum board.
Net sites selling sound absorbing/blocking material..whatever, should include how odorus it is.
On my attempt to quiet my HVAC hallway closet? I just pulled off $200 of net bought
fire retardant sound blocking/absorbing material. Why? It smells of year 2015 technology.
I have not yet determined if that fire retardant sound absorbing foam material made it's
smell into my system. Screw it. I yanked it out. Yanking it out took off the silver
backing of the previously installed 1" plenum board the installers put in.
I bought a 10x4 sheet of plenum board from my installer. I'm gonna go over my closet and
return intake with that.