Also happens when l turn the truck off. This just started yesterday and l am hoping it is not too major to fix or will cost a ton of cash.

You may have a kink in your exhaust system or you may have a leak in your muffler. You need to get under your explorer and have a good listen while the engine is idling.

Sounds as though u have a vacuum hose that has worn out & has come loose. Replacing it is cheap & quick. The problem is finding it. Look at all of the little black hoses on the engine to be sure that they r connected on both ends & r secure. If one has come completely off from where it should be connected it could be difficult to find where it belongs, so do not start pulling on hoses. Find the end of a hose & find its end & pull very gently on it to see if it is secure. If u find one that has completely come loose, do not touch it. Look near the loose end for the place it might belong. If u find it, cut off about 1/2 inch to an inch from the end (save it) & reconnect the hose. It should fit snugly. If it does not fit snugly, take the piece u cut off to an automotive store & buy some hose to replace the loose one. While u r at it, u might replace all of those little hoses (there r not a lot of them).

Good luck!

Go buy a can of starting fluid. Start engine and spray LITE shots at all hose connections. The leak will be revealed by an increase in engine RPM