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This noise is killing me!

I'm going to try to post this in a few boards because people are telling me it's not HVAC, but my husband and I are literally losing sleep over this so we need to find an answer.

To start, we live in Minneapolis, MN, and we bought the house in May 2015. The house has a forced-air central heating system. It was cool weather when we bought it, so I think we were still using the furnace on occasion. If I'm remembering correctly, we turned the a/c on for the first time in June. The a/c compressor has an on/off switch that was turned off when we bought the house. So we turned that on, and switched the thermostat to a/c. At some point shortly thereafter, we heard the noise for the first time. Sometimes we go weeks without hearing it. Sometimes it happens every hour (like today). It woke us up three times last night.

I hope this is allowed (and I hope the link works) -- I posted a short video of one of the high whistling sound. This is a publicly shared video on Google Drive: https://drive.google.com/file/d/0BzCXINv9QlCjTzJBejRGRlFkVlU/view?usp=sharing

Here are the facts we feel pretty sure of:

  • It happens during the warmer months, and maybe even on warmer days
  • It happens during the night and day but doesn't seem to be related to anything triggering it (happens when we're all asleep but also happens when we're all awake and using water, etc.)
  • It happens most often when the a/c system is on (thermostat set to cool), but has also happened when the system was off, but never happens with the a/c is running
  • Kicking the a/c on to blow air is the only thing that stops the noise
  • Sometimes it's a low moaning noise, sometimes it's a high whistling noise (like in the video), sometimes it's both
  • We've tried shutting off the main water valve but that doesn't stop it
  • The vibration can be felt when touching the furnace/ductwork in the basement
  • Most events last at least several minutes
  • We're on city water (not well) and we've had the water heater replaced in the time we've been here and it has not changed anything
  • We have a central humidifier but the valve is off in the summer

We had an HVAC technician come out twice. The first time he said it was a dirty filter (we have a crazy 20x20x4 filter). We replaced the filter but it made no difference. The second time he came out (in July), we were lucky enough to have him experience the noise first hand. It was happening frequently then and now we're back to July and it's going crazy again. The following is my husband's account of the conversation on the second visit:

So, the technician was unable to find the source of the noise. He said it was unrelated to the HVAC system, because the blower in the furnace wasn't running at the time the noise occurred. It's definitely something vibrating the trunk/ducting around the furnace, but he wasn't able to say what the cause was. There are some copper pipes running next to the furnace so he speculated that could be the cause, but he said it was just a guess.

I know this is a long post but I appreciate any help that can be offered. We don't know where to go from here. And thanks to those who have taken the time to read this whole post...hopefully someone has heard of a something similar before and has ideas for a solution! :) Kristine

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