So I've been warming my 4runner up in the morning here lately. The other morning i went out and cranked it up went back in side for about 5 minutes and when i came back out it was making some very loud noise. As i walked toward it, it sounded like whatever was making the noise cut off and the noise vanished. My only idea is it has to be something to do with my ac/heat cause it only does it when one or the other is blowing. The sound is like something tapping. I does not do it everytime the heat is on just sometimes. If i cut the heat off it will quit.