There is no way that it just appeared out of no where. They probably did a temporary fix that limited the noise and hoped it wouldn't reappear till after you drove it for 30 days or 3000 miles.. I would get it back to them asap and do you also have a period of time to return the truck? I once bought a truck from Cypress Coast ford in Monterey CA and they did that to me. But I had at least a 3 day window to return purchase which I did cause the first night of ownership the electrical system went haywire. I made some calls and luckily had a friend in the know with that stealership and he said return it immediately, because they new it had problems but could not fix it and since it was intermittent sold it to me hoping it wouldn't show problems till after my 3 day return window was up. Luckily for me it showed quickly but to learn dealerships do try to pass on problems is a ********* eye on the industry. Now I do my research on who I buy from, so hopefully your seller isn't like others and makes right.