What's making this noise?

[brucezas]

Every time my a/c goes on, something makes a BZZZT! sound for second. It seemed to start when my contractor reduced the speed of my blower motor. I searched on the internet, and I read about contact chatter which could apparently have a number of causes: dirty relay contacts, bad capacitor, bad transformer, or blower motor needing oil. Only I don't seem to hear this buzz when I'm standing right next to the air handler when it goes on, and I also don't seem to hear it when I'm standing outside next to the condenser when it goes on. I hear it when I'm in a room between the air handler and the condenser. It seems to happen every time, and it's loud enough to wake me up sometimes. I have also heard this noise when the a/c for a neighboring condo goes on, but I don't think I've ever heard it with my unit before now. I hung out in the attic between these units because some of the electrical goes through there, and I didn't hear it there either! WTF?

[ADillon]

It could be line set noise. The line set vibrating or refrigerant rushing through the lines. Are the refrigation lines running through/around the space where you are hearing the noise? You would know if your contactor was chattering.

[brucezas]

It could be line set noise. The line set vibrating or refrigerant rushing through the lines. Are the refrigation lines running through/around the space where you are hearing the noise?

Yes, they run under the floor of that room. I don't have access to them, except where they connect to the units, because they would be behind drywall of the garages below me. Anything I can do about it? Why would this have just started now? The contractor just reduced the speed of the blower motor and adjusted the charge. He did push on the ends of the lines outside a little bit checking for a possible leak, but not hard. Another contractor performed a leak test about 1 week earlier where he put the system under pressure and vacuum. Could that have caused it?

[brucezas]

The sound is coming from my circuit breaker box. Hopefully it will just be a matter of tightening the panel or replacing a breaker. I hope it's not because the compressor is having trouble starting because the charge isn't right again. Could setting the blower motor to a LOWER speed cause this?

[jtrammel]

Sounds fishy, call a tech and tell them what you have discovered. Could be hazardous

[pacnw]

I have heard this in a commercial building, turned it over to their electrician.

I a residential setting, it can be the wiring in the conduit quickly vibrating, but I do no know the cause.

If it is in the panel, get a qualified electrician out to check it.

[jtrammel]

Why did they lower speed of blower?

[brucezas]

Why did they lower speed of blower?

I had a contractor check my air pressures, and he said that my supply pressure was too high because the furnace my dealer sold me was bigger than I needed, and too big for my duct work and a-coil. He moved it from the highest speed to the lowest speed which reduced cfms from about 1000 to about 800, and he recommended a speed control to reduce it even more.

[Kevin Weaver]

I had a contractor check my air pressures, and he said that my supply pressure was too high because the furnace my dealer sold me was bigger than I needed, and too big for my duct work and a-coil. He moved it from the highest speed to the lowest speed which reduced cfms from about 1000 to about 800, and he recommended a speed control to reduce it even more.

it could be the high voltage breaker buzzing, seen this before.

[brucezas]

it could be the high voltage breaker buzzing, seen this before.

It is coming from inside the a/c breaker itself. I'll call the contractor to make sure the start up current hasn't become too high for some reason. I read that the buzzing means they are just on the threshold of blowing, and that when the breakers get old, they don't handle as much current. They are old, and the neighbor's have made the same noise for a long time with the same type of breakers. Apparently this buzzing is sometimes normal, but I've never had it before, and I'd like to get rid of it. If the a/c checks out, I'll replace the breaker with a new one. If a new 20 doesn't fix it, maybe I can use a 25 if it's safe.

I read where someone said that installing something called a "compressor hard start kit" can fix this problem.

Anyway, thanks to the guy that suggested that the line set was vibrating. That wasn't it, but listening to it got me in the right area to realize it's coming from the circuit breaker.