Heating, Air Conditioning, Fridge, HVAC/Baffling HVAC issue


QUESTION: Hi expert! I'm having a noise issue in my HVAC unit as follows:

I'm on the 5th floor of a 7th floor rental building (built 2009) with HVAC units. My unit is in a soffet with vents near the ceiling opening into two rooms on opposite sides of the wall. Since my move-in in March no problems until mid-June, where suddenly I started getting cyclical banging sounds from the ductwork. I assumed the problem was with my unit, but after testing, the sounds would happen even when my unit was totally turned off.  Specific details:

Sound is a sharp BANG, sounds like someone is slapping ductwork, or like something is hitting something inside the ductwork. It's a very discrete BANG sound and I can sense the sound coming from the soffet, from the top where the vents are.

The sound occurs  only after 3am through till about 8 am after which I leave for work. My mom (who's hard of hearing) was there once and heard it up till around 10 am. It is different than the normal A/C cycling - I get home and turn on the A/C and don't hear those bangs at all until the wee hours. Often it stops before 8 am but I have definitely heard it around 8:30 am a few times. It's keeping me up every night.

The sound seems to build up in severity over time. Sometimes it's irregular, but when it gets going it bangsregularly every 12-15 minutes, to the point I can predict when it will occur.

Since mid-June this has occurred regularly except for two periods of a week each where it stopped. Once, the upstairs neighbors were away: no noise for a week. The second time, the building replaced the upstairs unit (a week ago), no noise for a week, then it started again Saturday.

Yesterday they disconnected my unit totally, and a new unit is coming today, and last night the system still banged.

I am on the corner of the building with nobody on either side on one wall, one neighbor only on one wall of my unit.

The building can't figure it out, they haven't been able to be there to actually listen to the bang, I've sent them a recording, but I want them to bring in an HVAC pros, which they aren't. My friends and my Mom can testify to the noise.

So I'm awoken every night basically anywhere between 3-5 a.m., and the worst is when it's cycling every 12 minutes, as I lay in bed trying to go back to sleep, anticipating the next bang. It's torture, and going on 3 months. I don't know where to turn. They are trying to help but so far at a loss.

I can't move, so I'm reaching out for help - any advice appreciated thank you!  Barri

ANSWER: Is the a/c in your unit/building chillwater or refrigerant?  Is the bang just on quick sound like hitting something with a hammer or more like a tapping thumping sound?


---------- FOLLOW-UP ----------

QUESTION: it has chilled water towers, BAC towers, on the top of the building.  

The sound is a quick  bang, not quite like a hammer, but as if someone is slapping the ductwork, like the sound of vibrating metal.  Sounds like someone is in the duct and kicking it. I made a recording of the sound this morning.

 They supposedly changed the unit last night, same thing. Absolutely quiet until 5 am when it started banging regularly, getting louder and louder, in about 12-minute intervals.  I turned the unit off and the banging continues. But it's always silent when I get home.  It's such a mystery!  Thank you!

The problem does not appear to be coming from your unit.  To me the noise could be water hammer in the drinking water pipes or the a/c system pipes.  High rises normally have the piping running though a common chase and there is no telling what they could be rubbing up against causing the noise.  The noise would be more noticeable at night given the usage is down and things are much quieter.  


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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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