Intense Humming from Return air

[mrdenial]

We have a vaulted living room with a Return Air in the sidewall about 15' off the ground. This goes into the furnace that is in the attic. For the first 6 months, I didn't have return air grilles or supply grilles. During this time, the system didn't make any noises. Now, whenever the system turns on, I hear this whine/hum come from the return air. It's really intense when the heat strips kick on (I suppose because of the higher fan speed). During this time, I can sometimes hear the return grille vibrate.

I've gone into the attic a few times and I didn't feel any vibrations, so I'm thinking there is too much air being pulled through my returns. Does that sound like a decent guess?

I don't have a contractor to fall back on, so I'm trying to do as much leg work as possible.

Thanks!

[garya505]

Return air velocity is probably too high.

[beenthere]

Not enough returns. Or returns are too small.

[fearlessfurnace]

some grills are a lot more restrictive than others might be worth investigating. also blower could be set to high

[wgrr]

Harmonic vibration noise from the RA grills. Whithout looking at it that is as far as I can go. The suggestion that there is not enough return air could be correct.

[Stamas]

Enlarging/adding R/A and using a better R/A grill will probably cure that. Or do like my competitor did and put a pleated filter in, (already had a 4" Media air cleaner), NOT.

[Special-K]

I've seen this happen because the grill was actually vibrating against the wall. Putting a strip of self-adhesive foam tape between the grill and the wall can cure it in those cases.

I agree with the others though that you might be short on return which is causing the velocity of the airflow to be too high.

[Air Movers]

Def sounds like need more return, what size system is it, what size ductwork is pulling from it, what size is the grill, try a bar style you can see into it more but def is quieter. Hoe close is the air handler to that register if it is flex putting a loop or running the ductwork farther will also help.

[garya505]

If you could record a few notes of the humming sound maybe we could guess the song.

[Chris_Worthington]

Take a pair of needle nose pliers and straighten out the 45 degree fins on you're return grill, problem solved

[mrdenial]

3.5 ton N2H342 with variable speed blower

Here is a count of return air runs:

24x14 connected to 14" round flex - 21 foot to base can
8x8 connected to 6" flex - 26 foot to base can
12x12 connected to 10" flex - 26 foot to base can

[skibme]

Filter clean? I agree with all that posted so far.

[mrdenial]

How does the return air setup look based on my last post?

Filter is clean.

[beenthere]

How does the return air setup look based on my last post?

Not good.

[mrdenial]

Can someone help me calculate how much I need to add to my return air setup? I'm guessing each return duct has a certain amount of CFM it's able to handle which I can them compare to the CFM rating for my blower.

[bmb]

I've had some success with the needle-nose pliers. This is the most economical fix.

[bmb]

Unfortunately it sounds like you may need to hire a contractor.

[bmb]

each duct can deliver a certain (cfm's) amount of air. 90's and 45's restrict airflow, not to mention type of ducting. The amount of square inches of filter per ton of air conditioning is also important. This is why I recommend a professional.

[beenthere]

Your return grilles are all too small. To move the air your system needs, quietly. And your flex sizes are too small for quiet operation also.

24X14 grille should be 14X36(a 14X30 would flow air easier then the 24X14, and be less noise, how much, can't say)
8X8 should be a 14X14 with a 9" flex
12X12 should be a 14X20 with a 12" flex

[mrdenial]

Thanks for the replies.