When I turned my central heat/ac unit to heat this year, it started making a buzzing noise. If you turn the air off, then back on to heat-sometimes it will stop. Other times, it'll take 2-3 times to make it stop. What could be the problem? It doesn't do it all the time, and it only does it with the heat-not the ac.

It could be a relay making the sound or sometimes the low voltage transformer will also buzz.