i have a 99 5.9 durango with some mods . thought it was my tunner but its not when its not in gear and i rev the engine it doesnt rattle, but when its in gear it does around 3000 to 4500. sometimes it does it and some times it wont thought it was the rockers but its not thought it was the valves nope again. can it be the torque converter being lose or no. its driving me nuts

 

Since it's only when it's in gear, it has something to do with transmission. Could be something as simple as a band adjustment or the fluid going bad.

When was the last time the bands were adjusted? When was the last time the fluid was replaced with ATF +4?

 

I just did a fluid and filter change 2 months ago so the fluid is good I would try to make a band adj but thats one thing I have no clue on doing

 

if you have no clue how to do the band adjustment i would take it in and get that done, and also have them take a look over everything while its there, since they should have the trans open for this anyway it shouldnt be a big deal to just do a visual inspection for you