Funny, we just experienced the same thing this weekend during two hours of driving on a short camping trip - an intermittent and relatively quiet beep. I don't know if it's the same "beep" you experienced, and it took me awhile to figure out what it was, but I finally did. It was the Freightliner info center beeping to let me know that my "Check Engine" light had come on. It took me awhile to figure out what it was, because the light would go off almost immediately. It finally stayed on long enough for me to see what it was, and then it went off and stayed off.
It occurred three times over two hours of driving in 90 degree weather, on a combination of 35 mph curvy roads and 6% highway grades. The engine temp never even got close to being "hot". It was not a steady tone like the Jacks-Down warning, and nowhere near the same ear-piercing volume - just a little beep. I don't know if your QBH has the Powerglide or Freightliner chassis, and I don't even know if that would make any difference, but you might take a look at that. Our engine is the 450 horse Cummins.