I've noticed the past few days when I start my truck with the defrost on I hear a click about every 10 seconds and it idles up a little. Then another click after about another 10 seconds and it idles down a little. It only seems to be when I have the defrost on. With just the heat running it won't do it. It's been freezing cold here for the last week, so I haven't gotten a chance to see what it is that starts running. The fan clutch kicking in maybe? Either way it doesn't sound like it's supposed to be doing it. Any ideas?