I have a 2000 powerstroke and i get a strange noise when i let of the gas when traveling over 65 mph. the noise kinda sounds like exhaust or something of that nature but i am not sure. when i am on the gas, it goes away. it only happens if i am barely on the gas or let off and have to be going over 65mph. if i go 70mph and let of the gas the noise happens and as i drop below 65 mph it goes away. If i click the overdrive off, i do not get the noise over 65 mph. do you have any ideas what it could be?