Equipment - American Standard Freedom 80 Furnace. I recently had replaced flame sensor and tightened loose 24v transformer that was vibrating. Tightening the screws have solved the vibrating problems. Recently over the past few weeks I have experienced a whole new problem.

At night I turn the thermostat down to 65 deg. When I wake up in the morning, I will turn it up to 66 - 67 degrees to get the chill out of house. I have noticed this problem ONLY when I initially raise the temperature first thing in the morning. It will not happen any other then after the thermostat setting has been set lower overnight.

I turn up the thermostat. I hear the inducer motor start up. Then I hear the flame come on and heat up the furnace. Just before the blower kicks in, there will be 5 - 10 seconds of a loud buzzing or humming noise. The blower kick's in and the sound goes away. This may happen again day's later. I have been given 2 different answers to the problem. 1) I have been told it could be the capacitor, which makes more sense to me. 2) I have been told it is the 24 volt 35 va transformer. I am finding it hard to believe that little transformer could make that much noise. Note: That transformer is actually reading just under 30 volts instead of 24 volts it is rated for at 115 volt primary.

I put an amprobe on everything and can't seem to diagnose anything significant. NOTE : 2 years ago the Company that did the install was called out for a service call. Before they got there I found a ground between the 2 wires from the board and the compressor contactor. $7 in grounded wire cost me $ 1300. The repairman fried my board, and hooked up the blower motor back up wrong and fried that. In the end when everything was put back together, the problem ended up being in the $7 worth of wire, because the problem still existed after fixing everything they destroyed. Yes the owner did refund all my money. So the boards, transformer, and motor are only 2 yrs old. The furnace will be 9 yrs old this summer.

What's causing this intermittent buzzing / humming before the motor kicks in?

It sounds like the blower is struggling to start after perhaps being off for several hours during the night. Try this: set thermostat fan selector to ON at bedtime & see if the noise still occurs the next morning.

Thanks KC13 : I live in Chicago and we have had some pretty cold nights. I understand why you would lean toward the motor, but it isn't as if the furnace never comes on overnight at all. My thermostat is regularly set at 68 anyway. So it still goes on plenty overnight, as I stated it is much colder at night here.

I just don't think there is that much of a difference in the amount of times it actually goes on and off, between day's and evenings to warrant it only happening first thing in the morning. But is it possible that it could be a faulty motor capacitor? Is it possible that if the motor sits idle for a longer period, that the capacitor loses enough of a load to get it started during the first few seconds? That capacitor was put in with the motor. And as I said that motor is fairly new. I'd sure be pissed if it needs a new motor now.