I know that with my old Dodge when the Idler arm needed replaced it would do that at about 40 then go away. But after it got worse, it did it all the time. But I am guessing you are talking about a Ford. I am not sure how hot it is in AZ. right now but you might look for a buble in the tread of one of your tires. I have had that happen to where at slow speed you might not notice it. Just jack up one tire at a time about 1 inch off the ground then give it a spin. You should be able to spot it then if you watch the gap between the wheel and the ground. Other wise sometime they can be hard to spot untill
they are ready to blow. Next is to check for a bent rim that maybe got curbed or something like that.
Hope this helps. Kurt