Dripping Sound from Attic Furnace/AC?

[AlGreen]

Lately I've been hearing a dripping sound coming from one specific part of the ceiling in the second floor master bedroom. I've gone up to the attic (insulated) and looked for signs of a leak in the area, but can't find anything. There are no pipes that I can see under the plywood flooring.

The sound is, however, close to where the upstairs zone gas furnace and central air components are mounted, which includes some PVC piping to drain condensation outside.

I've read that PEX can move against wood and make a dripping sound. If that's true, I'm wondering if something similar could be happening with the PVC. Thoughts?

 

[Dana]

PEX against wood may make a ticking sound, but it's only rhythmic if it's changing temperature fairly rapidly.

Is it a condensing furnace?

 

[Sponsor]

 

[AlGreen]

It's an older Lennox natural gas furnace connected to one of two Carrier AC units outside.

 

[Dana]

Older can still be condensing (there are condensing Lennox units going back to the mid 1980s). Got a model number?

Hard-piped ducts can sometimes tick with expansion & contraction when heating up or cooling off too.

 

[AlGreen]

Older can still be condensing (there are condensing Lennox units going back to the mid 1980s). Got a model number?

Hard-piped ducts can sometimes tick with expansion & contraction when heating up or cooling off too.

I'll have to remove the panel tomorrow to find the model number. Here are a few shots of the setup in the meantime.

 

[Dana]

The B-vent exhaust venting is not designed for a condensing furnace, and it looks like the 80% efficiency Elite G50. I guess that rules out issues with the exhaust condensate management components (since it has none.) Unless the AC is running you shouldn't have any liquid in the AC condensate management plumbing & pan.

 

[AlGreen]

The B-vent exhaust venting is not designed for a condensing furnace, and it looks like the 80% efficiency Elite G50. I guess that rules out issues with the exhaust condensate management components (since it has none.) Unless the AC is running you shouldn't have any liquid in the AC condensate management plumbing & pan.

So it's a Lennox Elite G24M (X), though it sounds like that isn't a factor now.

I live in North NJ, and the AC has definitely been off when I've been hearing the noise. When I measured where the sound is approximately centered in the ceiling, the spot correlates with the area at the end of the tape measure.

The plywood isn't wet there, and when I pulled it up and moved the insulation aside, the plaster and lath ceiling looked dry. I'm baffled.

 

[MKS]

Is there a humidifier that maybe misbehaving?

 

[AlGreen]

Nope, we don't have a humdifier.

 

[WorthFlorida]

Might be some duct work or air register expanding or contracting. Even if there is no duct work where you hear it, it acts like a giant drum. Noise can propagate quite easily around the home.