The Trane CY5024A1060AB is a XL15c Packaged 2 ton Gas/Electric system. It incorporates a compressor for cooling and its the compressor whose vibration you feel. The compressor doesn't run during heating because the system uses gas for heating. That's why you only feel the vibration when the system is cooling. If you turn on the fan -- which runs during both cooling and heating -- by itself, you won't feel the vibration.

(I have a roof-mounted electric package heat pump that uses a compressor for both heating and cooling so I hear and feel the compressor's vibration whenever the system is running.)

-- amateur

Thanks so much for the comment, @wdccruise! I appreciate your insight! That does make sense, although the strange thing is the unit is outside the house on a pad on the ground (connected to the side of the house via a hood), and the main vibration on the floor is just above the distribution box. Additionally, the cool is 1-stage, but the heat is 2-stage, so I was wondering if that could contribute to the difference in vibration and why I went down the blower and not compressor route (i.e. when air is on the air return grates woosh loudly and floor vibrates, but when the heat is on you hear nothing).

According to the system's web page, the unit has "Two-stage gas heat" so the compressor doesn't run for heating; it only turns on for cooling.

I assume that the two round ducts attached to the "distribution box" in the photo lead to the two intake registers correct? An additional rigid duct connect the distribution box to the heat pump, right? I'll take a guess that compressor vibration is transmitted through that duct to the distribution box which is attached to the joist below the floor. Have you turned on the AC and felt the distribution box and the duct leading out to the heat pump to see if they are vibrating?

Finally, the fan in the heat pump may run faster during cooling than heating which is why you hear the woosh only during cooling.

-- amateur

Thank you for the idea as a quick sanity check @sktn77a!

Hi @wdccruise thanks for the info. Some of that is over my head, but indeed I have felt the components in the photo and confirmed the vibration so I am hoping if I test removing that box from the joists and it lowers felt vibration on the floor I'm on the right track. I appreciate the ideas and feedback!

"I am hoping if I test removing that box from the joists and it lowers felt vibration on the floor I'm on the right track."

There is probably rigid duct between the heat pump and the distribution box so you will not be able to simply detach the distribution box from the joists and move it.

@Mini-Split Warehouse: He doesn't have any of that stuff. He has a 2 ton (minimum size) package heat pump located outside his house.

^^^^^^^ Mr Minisplit's spamming bot missed the mark again!

lol @sktn77a

Last post since you both have been so helpful @sktn77a and @wdccruise I don't want to overstay my virtual welcome. :-) I am considering trying these out to see if it can help reduce distribution box and ductwork strap vibrations... sound reasonable? https://argco.com/boiler-hvac/2540170.html thanks!

You can certainly use those but I would experiment just having the box on the ground first. If that doesn't cure the problem, then the hangers won't.

Just to close the loop, I MacGyvered this setup lol so I wouldn't have to remove all duct straps and when decoupled from joists I can now barely feel any vibrations, so it was the distribution box. Thanks all for the ideas and help, much appreciated! Now to find a permanent fix.

The vibration hangers seem like a good idea.

We have attic-mounted A/C in two houses, and both the A/C unit and the ducts hang off the rafters. The supports for the A/C unit are metal, and the ducts runs hang off wide nylon fabric material. I'm sure these hangers are sold in any A/C supply house.

Bruce