Strange Tire Hum

So currently I am running BFG AT KOs at about 42psi.

I start to get noticeable hum around 65-70 mph and it just gets louder after that. Which is pretty normal, its not too loud either.

Here is the thing, at 75mph and only 75mph it goes dead quiet. I set my cruse control for 75 and I don't here a thing. Its not a problem, im just wondering what is going on here.

My last balance and alignment was about 2 months ago.

Anyone else have anything strange like this?

 

Way to much air in them..drop it down to 35 or 32 and test it out...

 

Well the max is 80psi, they are load E. I may put them down to 38psi and see how that is, if its better maybe I will go lower.