I have a lennox 13ACDA24 unit that has started making an intermittent humming noise.Varying time lapses but about every 30-45 seconds. Unit is operating and cooling and the humming continues when the compressor is running , again on 30-45 second intervals. Is this a two stage scroll compressor & does this make a differencen? We are new to this particular location, and have not heard this before.