This is copied from my reply on the RV site forum about the same subject. When sound waves crash into each other, they make strange noises...
I've heard this noise like a helicopter flapping overhead several times. As a long shot in the dark, I have to beleave it's accoustics of low frequency and high decibal sound reverberating off of the atmosphere. I have a db meter and have recorded sound waves too deep to hear but register 95db plus on the meter in the cab with the windows up. Our diesels are very loud in the low frequency ranges. While we hear the clatter at about 65-75db, the real noise is at lower hard to hear frequencies of 95+db. My Harley with straight pipes also does this at idle on hot humid days. From a distance it cackles but sitting on it gives that annoying helicopter bop. On cool dry days, it's a cackling ca-di-llac rythm. Ok, call my nuts!!!