just what the title says ive got a grinding noise from the front end of my blazer. i know it has something to do with the 4wd and im thinking but hoping its not the gears in the front diff. it started yesterday after i put the blazer in 4lo then took it out and drove it about 15 miles and at the end of my trip while i was going slower i heard what sounded like a moped under my hood. the slower i got the louder it got and as i slowed down i could feel a rappid nocking sound. i let it sit for a few hours and when i came back it didnt do it anymore then i went somewhere today and put it in 4wd to see if that was it but it didnt do it, then after i put it back in 2wd and drove about 50 feet i heard a pop and it started doing it again and now it wont start. any ideas?