Re: How can I reduce blower noise?
I agree, that more information would be helpful in trying to determine where the noise originates. Certainly insulation of the return cabinet would help; although insulation of the closet itself probably would not unless the noise was 'mechanical' in nature.
Sounds like you have a single return, which is the low-cost way of doing a return system and one of the problems as you are now aware, is that you are pulling the total amount of air in the system back through a single opening. Ideally, you would have a cold air return in each conditioned area of the house, with the exception of the kitchen and / or bathrooms.
If this cannot be achieved (for example if the house is built on a concrete slab) then you are probably limited to the single return.
You may want to create an offset of sorts on the return. In other words, raise the return grille to a high-wall location, and then put a duct riser flush with the interior of the closet and have it drop down to connect to the existing return of the furnace. With this offset, you will diffuse the sound and may find it quite acceptable.
Have a professional come out and look at your system and he can determine the feasibility of what I have proposed. I would not recommend that you attempt any of this yourself, so that in an effort to correct one problem, another is created.
All the best, Irishmist
Live your life as an EXCLAMATION; not an explanation!