Engine noise

We have a 2005 Chevy Trailblazer 73,000 miles it has a straight six (I hope there is only one size). When we start in the mornings or after it has sat for a couple of hours the engine makes a sound like a jet engine starting and makes this sound for about 10-20 seconds. It does this every day in the morning it does not seem to affect any thing and all the gages read fine. Let me know what you think.