Our furnace was making an intermittent vibrating noise that was thought to be the exhaust motor. The first service tech assumed it was that because it wasn't making the sound when he arrived, which was quite loud when it occurred. He ordered the part and it was replaced in two days or so by a different tech.It wasn't making the sound after replacement, so the tech left. Right after he left, the noise came back so another service call was scheduled. The same tech came back but it wasn't making the noise, but I had recorded the noise on my phone so that he could hear it. Another tech called me back the same night, and they both came out that same night and I let them hear the recording again. He turned the heat up to max to heat everything up, and the noise started. He said the problem is in the air con coil area in the plenum and the plan doesn't cover the duct work, but he offered some suggestions. At least we have the peace of mind knowing that it is just vibration from metal heat expansion. Great service...they demonstrated you really do care about your customers. Thanks!