Sizing of ductwork for upstairs system both supply and return should be evaluated. It should be sized for minimum of 1400-1500 CFMs. How many returns for upstairs system? What is size of living area for upstairs? Sizing of ductwork is likely cause especially since you were upsized.

Dealer did you a disservice by installing the XR series furnace. Should have been the XT furnace series at the very least.

BTW, source of hums can be located and identified by a good tech.

IMO

The upstairs is aprox. 1700 sq feet. With a huge master bathroom over a garage, which used to never get cool, now it does. We have six return air ducts, including the larger one installed with the new system. We do not plan on living here for more than five years so we wanted the least expensive option--I think that's why we got the XR, we hardly need heat where we live. But I wonder if that's part of the humming issue.

Every HVAC company that came in to quote said we had good duct work except for the master bath. Everyone said we needed an additional line to that, which is what we got. The old 2.5 ton Rudd never cooled the upstairs enough.

What would they do to correct the hum? Change out the air handler? The downstairs unit does not hum, it's a 2.5 ton. It's quiet outside now at night and it clicks on and all I hear is that annoying hum! Thank you for any opinion.

The problem is finding a good tech who wants to tackle ductwork! So you don't think the line about hardpipe humming is true? The company did say to get rid of the hum they can remove the hardpipe and put in flexible, but that the hardpipe is a better product. I really don't know what they are talking about.

The noise of the blower doesn't bother us, the Rudd was louder, it's the high pitched hum.

They are coming out again I will mention the transformer, and yes it only hums when it is on.

The house is only ten years old and it seems to be we have lots of vents, in the closets, baths, laundry room. But I guess it's the size of the duct pipes leading to those vents that matters. The main lead is huge in the attic.

And we don't have the hum with the smaller unit for the downstairs. thank you for your help.

Just in case someone is reading this post a resolution has been found to the humming! A senior service tech came to our house and adjusted the air exchanger fan so the speed isn't as high. He did his research to be sure the fan would still be at optimum for our system. Retested all the temps. Very good job and no more hum.

Thanks Diane for following up with the solution! We just moved into a new place with the same system and what seems to be the same problem. Going to try adjusting the fan speed down to see if it works.

Thank you again, random internet stranger, from 9 years later! :)