Shutting off that one might make the rest of them become loud as they suck harder.

Except for normal vent discharge openings into the bedrooms, there are no other HVAC return vents in the bedrooms except for that one.

Probably not a good idea. Blowers are chosen according to
the volume of space they need to condition and may work harder and become less
efficient when vents are closed. Maybe sound absorbing material in the duct or
on a screen near it. I'm not sure whether it was sprayed on or how it was
applied but my ducts are lined with a material that makes them very quiet.

I agree generally that you shouldn't close off return vents. But what I'm thinking here is it's a air handler in a closet and they're letting air into that closet from two sides - the hall on the one side and the bedroom on the other. Is that correct? If so the reason it's loud really comes not only from the air rushing through the return in the bedroom but also the fact that having that return there opens up the bedroom to that closet that contains the air handler/furnace, if my vision of this is right. If this is true and I wanted to close off the bedroom vent I'd make the hallway one bigger by at least the same amount of space.

Another thing to think about is if it's loud, airflow might already be fairly restricted and you might want to think about increasing the return air volume in other ways too.

Unless someone that knows the capacity of your furnace/AC and knows how to do the calculations says that it is OK, it is not OK. It is very likely not OK. The bedroom intake is 40% of the total. What you'd like to do would be like trying to breathe through only the better nostril stopping off the more constricted side. Try it and see how your blower "feels" :-)

If the bedroom return is closed off, you have to provide another way for any return air entering there to get back to the blower. Is that just from one room or is there an other area involved?

This is another reason why cramming a system in a closet inside the house envelope is not what it's cracked up to be.

Design problems, are big problems that typically have to be redesigned. This may mean relocating the system. You can only do so much with a bad system design.

Well I just guessed that, as far as the closet location, based on the original poster's description referring to the hallway and bedroom "opening to the furnace," so it sounded like letting air into the room rather than actual return ducts.

If Mike C is still reading, please post again and provide some additional information. If my description is accurate, you could increase the hallway opening to make up for the bedroom one, possibly expand it even more than what you propose to close off, but at the least you need to expand it by as much as you're eliminating. If there's not enough air getting into the closet (if that's what this is) that contributes to making it loud, and it can also cause it to draw air from the attic, crawl space or other unconditioned areas, making the system less efficient and less effective.

The bedroom hole (17 X 14) is providing a secondary return plenum for the AC. The main return (25 X 14) is in the hallway. The plenum is formed by the box the furnace sits on. The two returns enter into that box. There are only those two returns for the whole house. The worst return noise is from the bedroom hole, it also provides no sound privacy to any occupants in the room when the AC isn't running - voices go right into hallway/living room.

kenjwy - I looked into making the hallway entrance as large as the furnace's 25" X 20" intake, but structure is in the way, can't go wider or taller.

Thanks. I think then that you'd probably need to do as Austin suggests and get the system modified, possibly bring in some return air from somewhere else.

One other thing just possibly to look at - are you using one of the "high-efficiency" filters that are advertised to take out pollen and such? If so get rid of it and put in a standard (cheap) filter. The expensive filters are restrictive and cause more noise.

One other thing I was thinking: The issue here is static pressure. An HVAC company could measure the static pressure in that plenum and advise you of whether closing off the one side matters. It could, but it also might not when you consider that most systems have a network of return ducts the blower has to pull air through. You would have one opening directly below the furnace, basically. If you had five returns throughout the house connected by long duct runs, I could envision that the static pressure below that furnace might be higher than with the plenum just opening up to the hall right there.

The answer is knowable with testing, that's all I'm saying. I know it is quite common to have return air coming to a furnace in a closet through one return.

Modification, no. Redesign yes.

Based on what the OP describes as the problem, the only true way he is going to be satisfied is likely going to need a major redesign of this HVAC system.

This could include upgrading the system to a quieter furnace as well as relocating the system elsewhere and redesigning and installing a new duct system.

There is only so much you can do with what you have. If there is no better place to relocate the indoor equipment to, you may have just discovered the reason this house was sold to you.

It is very common for people to run from problems as opposed to fixing them.

I would think so if it's just a bridge vent.

http://www.heating-repair-chicago.com

You should post a picture we could better tell you is it connected with duct work or is it just a connection event or bridge then so to speak?

http://www.heating-repair-chicago.com

Thanks for all the comments. House is 38 years old, that's always been where HVAC was located. Not cost effective to redesign the installation. Its a 'standard' heating/cooling closet with intake plenum under the furnace with all ductwork located in the attic.

All you need is to have someone check the static pressure with a manometer. There are a thousand things you could do, or that would be nice to do, but that's the only way I know of that you can know whether closing off the one side would be a problem.

If you can introduce another opening or two in the return(s), that would be helpful. Mine was bottle-necked as well. I introduced a floor grille upstairs and two wall grills (on the ceiling) downstairs. My returns use joist space so the capacity is there for air travel, it just needs the correct sq in of opening. Previous opening were returns on the wall and bottle necked to the width of the wall as it goes through the floor (4" wide).

Once you travel down this rabbit hole, these are the guidelines I use for quiet air flow. You can research how to adjust fan speed in the air handler as well. If you look up capacities, you will find something like 80 sq inch per 400 cfm, which will be loud in my opinion. 100 sq in per 400 cfm is what a lot use, but that also is loud in my opinion.

200 sq in per ton
200 sq in per 400 CFM
400 CFM per ton
600 CFM per 40000 BTU