We have just had two new forced air gas furnaces installed. Both are two-stage variable speed units, with single stage t-stats. One furnace services the bottom two floors of the house and is 75,000 btus, and the other furnace services the top two floors and is 50,000 btus. The unit for the bottom two floors works perfectly. Unfortunately, the unit for the top two floors has been creating a loud resonating noise, which can be VERY annoying. The installer has been very good, and he has offered to come and replace the motor if necessary. But first he wanted to try some trouble shooting. Makes sense to me. He spoke with the tech support people at the manufacturer, and they suggested adjusting the some dip switches to change blower speeds, so I have been working with the installer to do just this. The factory default for the furnace had a Lo heat speed of about 625cfm, and a hi heat speed of about 825cfm (both at 0.1 of static pressure). This was the setting that was originally producing the loud resonating sound. Over a few days I have tried lower and lower blower speeds in an attempt to eliminate the noise. I have now gone as low as it will go, with a Lo heat speed of 500cfm and a hi heat speed of about 675cfm. At this point the noise at lo speed is completely gone, but instead there is some noise at hi speed. I should note that with the factory settings the noise occurred at low speed. This leaves me to believe that the "noisy" range is from about 675 to 825cfm. Unfortunately, it is not possible to adjust the lo heat and hi heat speeds independently, so it doesn't seem like I can come up with a totally quiet combination, although the way it is set now is considerably better. My question is - is a new motor likely to fix the problem? If we opt to leave it as it is now (ie. 500cfm/675cfm), will it function properly? It seems that at lo speed there is very little air movement out of the vents. Should I be concerned about this? Am I compromising efficiency at this low blower speed? Any input is much appreciated.